lavfi: add a new struct for private link properties

Specifically those that should be visible to filters, but hidden from
API callers. Such properties are currently located at the end of the
public AVFilterLink struct, demarcated by a comment marking them as
private. However it is generally better to hide them explicitly, using
the same pattern already employed in avformat or avcodec.

The new struct is currently trivial, but will become more useful in
following commits.

libavfilter/avfilter_internal.h

@@ -28,10 +28,11 @@
 #include <stdint.h>
 
 #include "avfilter.h"
+#include "filters.h"
 #include "framequeue.h"
 
 typedef struct FilterLinkInternal {
-    AVFilterLink l;
+    FilterLink l;
 
     struct FFFramePool *frame_pool;