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/stdlib/SDL_crc16.c

@@ -20,7 +20,6 @@
 */
 #include "../SDL_internal.h"
 
-#include "SDL_stdinc.h"
 
 
 /* Public domain CRC implementation adapted from:

src/stdlib/SDL_crc32.c

@@ -20,7 +20,6 @@
 */
 #include "../SDL_internal.h"
 
-#include "SDL_stdinc.h"
 
 
 /* Public domain CRC implementation adapted from:

src/stdlib/SDL_getenv.c

@@ -33,7 +33,6 @@
 #include "../core/android/SDL_android.h"
 #endif
 
-#include "SDL_stdinc.h"
 
 #if (defined(__WIN32__) || defined(__WINGDK__)) && (!defined(HAVE_SETENV) || !defined(HAVE_GETENV))
 /* Note this isn't thread-safe! */

src/stdlib/SDL_iconv.c

@@ -27,8 +27,6 @@
 
 /* This file contains portable iconv functions for SDL */
 
-#include "SDL_stdinc.h"
-#include "SDL_endian.h"
 
 #if defined(HAVE_ICONV) && defined(HAVE_ICONV_H)
 #ifdef __FreeBSD__

src/stdlib/SDL_malloc.c

@@ -26,9 +26,6 @@
 #include "../SDL_internal.h"
 
 /* This file contains portable memory management functions for SDL */
-#include "SDL_stdinc.h"
-#include "SDL_atomic.h"
-#include "SDL_error.h"
 
 #ifndef HAVE_MALLOC
 #define LACKS_SYS_TYPES_H

src/stdlib/SDL_qsort.c

@@ -25,7 +25,6 @@
 
 #include "../SDL_internal.h"
 
-#include "SDL_stdinc.h"
 
 #if defined(HAVE_QSORT)
 void

src/stdlib/SDL_stdlib.c

@@ -27,7 +27,6 @@
 
 /* This file contains portable stdlib functions for SDL */
 
-#include "SDL_stdinc.h"
 #include "../libm/math_libm.h"
 
 

src/stdlib/SDL_strtokr.c

@@ -24,7 +24,6 @@
 
 #include "../SDL_internal.h"
 
-#include "SDL_stdinc.h"
 
 char *SDL_strtokr(char *s1, const char *s2, char **ptr)
 {