include: Always declare SDL_GDKSuspendComplete.

This is always defined in the symbol table and the function safely does nothing
on non-GDK targets, so treat it like any other SDL_system function.

include/SDL3/SDL_main.h

@@ -567,17 +567,16 @@ extern SDL_DECLSPEC void SDLCALL SDL_UnregisterApp(void);
 
 #endif /* defined(SDL_PLATFORM_WINDOWS) */
 
-#ifdef SDL_PLATFORM_GDK
-
 /**
  * Callback from the application to let the suspend continue.
  *
+ * This function is only needed for Xbox GDK support; all other platforms will
+ * do nothing and set an "unsupported" error message.
+ *
  * \since This function is available since SDL 3.1.3.
  */
 extern SDL_DECLSPEC void SDLCALL SDL_GDKSuspendComplete(void);
 
-#endif /* SDL_PLATFORM_GDK */
-
 #ifdef __cplusplus
 }
 #endif