Renaming of guard header names to quiet -Wreserved-id-macro

Patch contributed by Sylvain

include/SDL_quit.h

@@ -25,8 +25,8 @@
  *  Include file for SDL quit event handling.
  */
 
-#ifndef _SDL_quit_h
-#define _SDL_quit_h
+#ifndef SDL_quit_h_
+#define SDL_quit_h_
 
 #include "SDL_stdinc.h"
 #include "SDL_error.h"
@@ -55,4 +55,4 @@
 #define SDL_QuitRequested() \
         (SDL_PumpEvents(), (SDL_PeepEvents(NULL,0,SDL_PEEKEVENT,SDL_QUIT,SDL_QUIT) > 0))
 
-#endif /* _SDL_quit_h */
+#endif /* SDL_quit_h_ */