lavfi: make FFERROR_NOT_READY available to filters.

I am not entirely sure that this return code is useful,
but having and using it makes no harm.

libavfilter/avfilter.c

@@ -1304,8 +1304,6 @@ static int forward_status_change(AVFilterContext *filter, AVFilterLink *in)
     return 0;
 }
 
-#define FFERROR_NOT_READY FFERRTAG('N','R','D','Y')
-
 static int ff_filter_activate_default(AVFilterContext *filter)
 {
     unsigned i;

libavfilter/filters.h

@@ -27,6 +27,11 @@
 
 #include "avfilter.h"
 
+/**
+ * Special return code when activate() did not do anything.
+ */
+#define FFERROR_NOT_READY FFERRTAG('N','R','D','Y')
+
 /**
  * Mark a filter ready and schedule it for activation.
  *