Update dummy_video to have the correct surface signature.

I forgot to update dummy_video.cpp in commit
0c27eb9490, which causes travis compiles
to fail.

src/dialogs.cpp

@@ -715,7 +715,7 @@ void save_preview_pane::draw_contents()
 		return;
 	}
 
-	surface &screen = video().getSurface();
+	surface& screen = video().getSurface();
 
 	SDL_Rect const &loc = location();
 	const SDL_Rect area = sdl::create_rect(loc.x + save_preview_border

src/tools/dummy_video.cpp

@@ -33,7 +33,7 @@ surface display_format_alpha(surface)
 	return NULL;
 }
 
-surface get_video_surface()
+surface& get_video_surface()
 {
 	return NULL;
 }