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/libm/k_rem_pio2.c

@@ -128,7 +128,6 @@
 #include "math_libm.h"
 #include "math_private.h"
 
-#include "SDL_assert.h"
 
 static const int init_jk[] = {2,3,4,6}; /* initial value for jk */
 

src/libm/math_private.h

@@ -18,7 +18,6 @@
 #define _MATH_PRIVATE_H_
 
 /* #include <endian.h> */
-#include "SDL_endian.h"
 /* #include <sys/types.h> */
 
 #define _IEEE_LIBM