adds GDK suspend/resume basic handling (#6596)

(cherry picked from commit f6144dfe03)

include/SDL_main.h

@@ -254,6 +254,12 @@ extern DECLSPEC int SDLCALL SDL_UIKitRunApp(int argc, char *argv[], SDL_main_fun
  */
 extern DECLSPEC int SDLCALL SDL_GDKRunApp(SDL_main_func mainFunction, void *reserved);
 
+/**
+ * Callback from the application to let the suspend continue.
+ *
+ */
+extern DECLSPEC void SDLCALL SDL_GDKSuspendComplete(void);
+
 #endif /* __GDK__ */
 
 #ifdef __cplusplus