Only list libavfilter filters if libavfilter has been enabled during

configuration.
Fix compilation if FFmpeg is not configured with --enable-avfilter.

Originally committed as revision 20813 to svn://svn.ffmpeg.org/ffmpeg/trunk

cmdutils.c

@@ -575,8 +575,10 @@ void show_filters(void)
     AVFilter **filter = NULL;
 
     printf("Filters:\n");
+#if CONFIG_AVFILTER
     while ((filter = av_filter_next(filter)) && *filter)
         printf("%-16s %s\n", (*filter)->name, (*filter)->description);
+#endif
 }
 
 int read_yesno(void)