remove warning prototype SDL_ExitProcess()

src/SDL.c

@@ -56,6 +56,7 @@ extern int SDL_HelperWindowDestroy(void);
 /* This is not declared in any header, although it is shared between some
     parts of SDL, because we don't want anything calling it without an
     extremely good reason. */
+extern SDL_NORETURN void SDL_ExitProcess(const int exitcode);
 SDL_NORETURN void SDL_ExitProcess(const int exitcode)
 {
 #ifdef __WIN32__