Fixed declaration of SDL_main_func for C++

include/SDL_main.h

@@ -118,7 +118,10 @@
 /**
  *  The prototype for the application's main() function
  */
-typedef C_LINKAGE SDLMAIN_DECLSPEC int (*SDL_main_func)(int argc, char *argv[]);
+#ifdef __cplusplus
+extern "C"
+#endif
+typedef int (*SDL_main_func)(int argc, char *argv[]);
 extern C_LINKAGE SDLMAIN_DECLSPEC int SDL_main(int argc, char *argv[]);