lavfi: add ashowinfo filter

Useful for debugging.

libavfilter/allfilters.c

@@ -37,6 +37,7 @@ void avfilter_register_all(void)
     REGISTER_FILTER (AFORMAT,     aformat,     af);
     REGISTER_FILTER (ANULL,       anull,       af);
     REGISTER_FILTER (ARESAMPLE,   aresample,   af);
+    REGISTER_FILTER (ASHOWINFO,   ashowinfo,   af);
 
     REGISTER_FILTER (ANULLSRC,    anullsrc,    asrc);