SDL.c (SDL_ExitProcess): unconstify its param to match its declaration.

src/SDL.c

@@ -57,7 +57,7 @@ extern int SDL_HelperWindowDestroy(void);
     parts of SDL, because we don't want anything calling it without an
     extremely good reason. */
 extern SDL_NORETURN void SDL_ExitProcess(int exitcode);
-SDL_NORETURN void SDL_ExitProcess(const int exitcode)
+SDL_NORETURN void SDL_ExitProcess(int exitcode)
 {
 #ifdef __WIN32__
     /* "if you do not know the state of all threads in your process, it is