Fixed bug 3745 - specify SDLCALL as the calling convention for API callbacks

Patches contributed by Ozkan Sezer

include/SDL_thread.h

@@ -273,7 +273,7 @@ extern DECLSPEC void * SDLCALL SDL_TLSGet(SDL_TLSID id);
  *  \sa SDL_TLSCreate()
  *  \sa SDL_TLSGet()
  */
-extern DECLSPEC int SDLCALL SDL_TLSSet(SDL_TLSID id, const void *value, void (*destructor)(void*));
+extern DECLSPEC int SDLCALL SDL_TLSSet(SDL_TLSID id, const void *value, void (SDLCALL *destructor)(void*));
 
 
 /* Ends C function definitions when using C++ */