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/timer/SDL_timer.c

@@ -20,10 +20,7 @@
 */
 #include "../SDL_internal.h"
 
-#include "SDL_timer.h"
 #include "SDL_timer_c.h"
-#include "SDL_atomic.h"
-#include "SDL_cpuinfo.h"
 #include "../thread/SDL_systhread.h"
 
 /* #define DEBUG_TIMERS */

src/timer/SDL_timer_c.h

@@ -25,7 +25,6 @@
 #include "../SDL_internal.h"
 
 /* Useful functions and variables from SDL_timer.c */
-#include "SDL_timer.h"
 
 #define ROUND_RESOLUTION(X) \
     (((X+TIMER_RESOLUTION-1)/TIMER_RESOLUTION)*TIMER_RESOLUTION)

src/timer/dummy/SDL_systimer.c

@@ -22,7 +22,6 @@
 
 #if defined(SDL_TIMER_DUMMY) || defined(SDL_TIMERS_DISABLED)
 
-#include "SDL_timer.h"
 
 static SDL_bool ticks_started = SDL_FALSE;
 

src/timer/haiku/SDL_systimer.c

@@ -24,7 +24,6 @@
 
 #include <kernel/OS.h>
 
-#include "SDL_timer.h"
 
 static bigtime_t start;
 static SDL_bool ticks_started = SDL_FALSE;

src/timer/ngage/SDL_systimer.cpp

@@ -25,7 +25,6 @@
 #include <e32std.h>
 #include <e32hal.h>
 
-#include "SDL_timer.h"
 
 static SDL_bool ticks_started = SDL_FALSE;
 static TUint    start         = 0;

src/timer/ps2/SDL_systimer.c

@@ -22,9 +22,6 @@
 
 #ifdef SDL_TIMER_PS2
 
-#include "SDL_thread.h"
-#include "SDL_timer.h"
-#include "SDL_error.h"
 #include "../SDL_timer_c.h"
 #include <stdlib.h>
 #include <time.h>

src/timer/psp/SDL_systimer.c

@@ -22,9 +22,6 @@
 
 #ifdef SDL_TIMER_PSP
 
-#include "SDL_thread.h"
-#include "SDL_timer.h"
-#include "SDL_error.h"
 #include "../SDL_timer_c.h"
 #include <stdlib.h>
 #include <time.h>

src/timer/unix/SDL_systimer.c

@@ -27,8 +27,6 @@
 #include <unistd.h>
 #include <errno.h>
 
-#include "SDL_timer.h"
-#include "SDL_hints.h"
 #include "../SDL_timer_c.h"
 
 #ifdef __EMSCRIPTEN__

src/timer/vita/SDL_systimer.c

@@ -22,9 +22,6 @@
 
 #ifdef SDL_TIMER_VITA
 
-#include "SDL_thread.h"
-#include "SDL_timer.h"
-#include "SDL_error.h"
 #include "../SDL_timer_c.h"
 #include <stdlib.h>
 #include <time.h>

src/timer/windows/SDL_systimer.c

@@ -25,8 +25,6 @@
 #include "../../core/windows/SDL_windows.h"
 #include <mmsystem.h>
 
-#include "SDL_timer.h"
-#include "SDL_hints.h"
 
 
 /* The first (low-resolution) ticks value of the application */