Fixed warning: this function declaration is not a prototype

src/video/SDL_sysvideo.h

@@ -544,7 +544,7 @@ extern void SDL_OnWindowFocusGained(SDL_Window *window);
 extern void SDL_OnWindowFocusLost(SDL_Window *window);
 extern void SDL_OnWindowDisplayChanged(SDL_Window *window);
 extern void SDL_UpdateWindowGrab(SDL_Window *window);
-extern SDL_Window *SDL_GetToplevelForKeyboardFocus();
+extern SDL_Window *SDL_GetToplevelForKeyboardFocus(void);
 
 extern SDL_bool SDL_ShouldAllowTopmost(void);
 

src/video/SDL_video.c

@@ -3561,7 +3561,7 @@ void SDL_OnWindowFocusLost(SDL_Window *window)
     }
 }
 
-SDL_Window *SDL_GetToplevelForKeyboardFocus()
+SDL_Window *SDL_GetToplevelForKeyboardFocus(void)
 {
     SDL_Window *focus = SDL_GetKeyboardFocus();