Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

src/filesystem/android/SDL_sysfilesystem.c

@@ -27,9 +27,6 @@
 
 #include <unistd.h>
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
-#include "SDL_system.h"
 
 
 char *

src/filesystem/cocoa/SDL_sysfilesystem.m

@@ -29,9 +29,6 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/dummy/SDL_sysfilesystem.c

@@ -25,8 +25,6 @@
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 /* System dependent filesystem routines                                */
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/emscripten/SDL_sysfilesystem.c

@@ -27,8 +27,6 @@
 #include <errno.h>
 #include <sys/stat.h>
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
 
 #include <emscripten/emscripten.h>
 

src/filesystem/haiku/SDL_sysfilesystem.cc

@@ -30,9 +30,6 @@
 #include <storage/Entry.h>
 #include <storage/Path.h>
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/n3ds/SDL_sysfilesystem.c

@@ -29,8 +29,6 @@
 #include <dirent.h>
 #include <errno.h>
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
 
 SDL_FORCE_INLINE char *MakePrefPath(const char *app);
 SDL_FORCE_INLINE int CreatePrefPathDir(const char *pref);

src/filesystem/ps2/SDL_sysfilesystem.c

@@ -28,8 +28,6 @@
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 /* System dependent filesystem routines                                */
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/psp/SDL_sysfilesystem.c

@@ -28,8 +28,6 @@
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 /* System dependent filesystem routines                                */
 
-#include "SDL_error.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/riscos/SDL_sysfilesystem.c

@@ -29,9 +29,6 @@
 #include <swis.h>
 #include <unixlib/local.h>
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
 
 /* Wrapper around __unixify_std that uses SDL's memory allocators */
 static char *

src/filesystem/unix/SDL_sysfilesystem.c

@@ -38,10 +38,6 @@
 #include <sys/sysctl.h>
 #endif
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
-#include "SDL_rwops.h"
 
 static char *
 readSymLink(const char *path)

src/filesystem/vita/SDL_sysfilesystem.c

@@ -34,10 +34,6 @@
 #include <limits.h>
 #include <fcntl.h>
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
-#include "SDL_rwops.h"
 
 char *
 SDL_GetBasePath(void)

src/filesystem/windows/SDL_sysfilesystem.c

@@ -28,9 +28,6 @@
 #include "../../core/windows/SDL_windows.h"
 #include <shlobj.h>
 
-#include "SDL_error.h"
-#include "SDL_stdinc.h"
-#include "SDL_filesystem.h"
 
 char *
 SDL_GetBasePath(void)
@@ -173,8 +170,6 @@ SDL_GetPrefPath(const char *org, const char *app)
 #endif /* SDL_FILESYSTEM_WINDOWS */
 
 #ifdef SDL_FILESYSTEM_XBOX
-#include "SDL_filesystem.h"
-#include "SDL_error.h"
 char *
 SDL_GetBasePath(void)
 {

src/filesystem/winrt/SDL_sysfilesystem.cpp

@@ -26,11 +26,6 @@
 #ifdef __WINRT__
 
 extern "C" {
-#include "SDL_filesystem.h"
-#include "SDL_error.h"
-#include "SDL_hints.h"
-#include "SDL_stdinc.h"
-#include "SDL_system.h"
 #include "../../core/windows/SDL_windows.h"
 }