lavfi: fix registration name for the buffersink sink

Previously both the buffer and buffersink elements were associated to
the "BUFFER" symbol in allfilters.c, so it was not possible to enable
one without the other.

libavfilter/allfilters.c

@@ -89,6 +89,6 @@ void avfilter_register_all(void)
     REGISTER_FILTER (RGBTESTSRC,  rgbtestsrc,  vsrc);
     REGISTER_FILTER (TESTSRC,     testsrc,     vsrc);
 
-    REGISTER_FILTER (BUFFER,      buffersink,  vsink);
+    REGISTER_FILTER (BUFFERSINK,  buffersink,  vsink);
     REGISTER_FILTER (NULLSINK,    nullsink,    vsink);
 }