lavfi/internal: move functions used by filters to filters.h

internal.h currently mixes interfaces intended to be used by filters
with those that should be limited to generic filter- or graph-level
code.

libavfilter/avfiltergraph.c

@@ -36,6 +36,7 @@
 #include "avfilter.h"
 #include "avfilter_internal.h"
 #include "buffersink.h"
+#include "filters.h"
 #include "formats.h"
 #include "framequeue.h"
 #include "internal.h"