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/thread/pthread/SDL_syscond.c

@@ -26,7 +26,6 @@
 #include <errno.h>
 #include <pthread.h>
 
-#include "SDL_thread.h"
 #include "SDL_sysmutex_c.h"
 
 struct SDL_cond

src/thread/pthread/SDL_sysmutex.c

@@ -23,7 +23,6 @@
 #include <errno.h>
 #include <pthread.h>
 
-#include "SDL_thread.h"
 
 #if !SDL_THREAD_PTHREAD_RECURSIVE_MUTEX && \
     !SDL_THREAD_PTHREAD_RECURSIVE_MUTEX_NP

src/thread/pthread/SDL_syssem.c

@@ -26,8 +26,6 @@
 #include <sys/time.h>
 #include <time.h>
 
-#include "SDL_thread.h"
-#include "SDL_timer.h"
 
 /* Wrapper around POSIX 1003.1b semaphores */
 

src/thread/pthread/SDL_systhread.c

@@ -20,8 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#include "SDL_system.h"
-#include "SDL_hints.h"
 
 #include <pthread.h>
 
@@ -48,7 +46,6 @@
 #endif
 #endif
 
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #ifdef __ANDROID__

src/thread/pthread/SDL_systls.c

@@ -19,7 +19,6 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 #include "../../SDL_internal.h"
-#include "SDL_thread.h"
 #include "../SDL_systhread.h"
 #include "../SDL_thread_c.h"