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/af_crystalizer.c

@@ -22,6 +22,7 @@
 #include "libavutil/opt.h"
 #include "avfilter.h"
 #include "audio.h"
+#include "filters.h"
 
 typedef struct CrystalizerContext {
     const AVClass *class;