ffplay: mark i variable in toggle_full_screen() as unused

Fix gcc warning:
ffplay.c: In function ‘toggle_full_screen’:
ffplay.c:2792:9: warning: unused variable ‘i’ [-Wunused-variable]

ffplay.c

@@ -2769,7 +2769,7 @@ static void stream_cycle_channel(VideoState *is, int codec_type)
 
 static void toggle_full_screen(VideoState *is)
 {
-    int i;
+    av_unused int i;
     is_full_screen = !is_full_screen;
 #if defined(__APPLE__) && SDL_VERSION_ATLEAST(1, 2, 14)
     /* OS X needs to reallocate the SDL overlays */