Address a compiler warning.

src/video/SDL_sysvideo.h

@@ -403,7 +403,7 @@ extern void *SDL_GetDisplayDriverData( int displayIndex );
 extern void SDL_GL_DeduceMaxSupportedESProfile(int* major, int* minor);
 
 extern int SDL_RecreateWindow(SDL_Window * window, Uint32 flags);
-extern SDL_bool SDL_HasWindows();
+extern SDL_bool SDL_HasWindows(void);
 
 extern void SDL_OnWindowShown(SDL_Window * window);
 extern void SDL_OnWindowHidden(SDL_Window * window);

src/video/SDL_video.c

@@ -1609,7 +1609,7 @@ SDL_RecreateWindow(SDL_Window * window, Uint32 flags)
 }
 
 SDL_bool
-SDL_HasWindows()
+SDL_HasWindows(void)
 {
     return (_this && _this->windows != NULL);
 }