Mark conditionally used variable as av_unused, fixes the warning:

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


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20943 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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