Mark conditionally used variable as av_unused, fixes the warning:

cmdutils.c:576: warning: unused variable ‘filter’

Originally committed as revision 20943 to svn://svn.ffmpeg.org/ffmpeg/trunk

cmdutils.c

@@ -573,7 +573,7 @@ void show_protocols(void)
 
 void show_filters(void)
 {
-    AVFilter **filter = NULL;
+    AVFilter av_unused(**filter) = NULL;
 
     printf("Filters:\n");
 #if CONFIG_AVFILTER