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

Patch contributed by Sylvain

include/SDL_config_winrt.h

@@ -19,8 +19,8 @@
   3. This notice may not be removed or altered from any source distribution.
 */
 
-#ifndef _SDL_config_winrt_h
-#define _SDL_config_winrt_h
+#ifndef SDL_config_winrt_h_
+#define SDL_config_winrt_h_
 
 #include "SDL_platform.h"
 
@@ -211,4 +211,4 @@ typedef unsigned int uintptr_t;
 #define SDL_ASSEMBLY_ROUTINES	1
 #endif
 
-#endif /* _SDL_config_winrt_h */
+#endif /* SDL_config_winrt_h_ */