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/windows/SDL_syscond_cv.c

@@ -20,8 +20,6 @@
 */
 #include "../../SDL_internal.h"
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
 
 #include "../generic/SDL_syscond_c.h"
 #include "SDL_sysmutex_c.h"

src/thread/windows/SDL_sysmutex.c

@@ -31,7 +31,6 @@
  */
 
 
-#include "SDL_hints.h"
 
 #include "SDL_sysmutex_c.h"
 

src/thread/windows/SDL_sysmutex_c.h

@@ -22,7 +22,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_mutex.h"
 
 typedef SDL_mutex * (*pfnSDL_CreateMutex)(void);
 typedef int (*pfnSDL_LockMutex)(SDL_mutex *);

src/thread/windows/SDL_syssem.c

@@ -35,9 +35,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
-#include "SDL_timer.h"
 
 typedef SDL_sem * (*pfnSDL_CreateSemaphore)(Uint32);
 typedef void (*pfnSDL_DestroySemaphore)(SDL_sem *);

src/thread/windows/SDL_systhread.c

@@ -24,8 +24,6 @@
 
 /* Win32 thread management routines for SDL */
 
-#include "SDL_hints.h"
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 #include "../SDL_systhread.h"
 #include "SDL_systhread_c.h"

src/thread/windows/SDL_systls.c

@@ -25,7 +25,6 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-#include "SDL_thread.h"
 #include "../SDL_thread_c.h"
 
 #if WINAPI_FAMILY_WINRT