avcodec/avpacket: add av_packet_make_refcounted()

It works as a drop in replacement for the deprecated av_dup_packet(),
to ensure a packet is reference counted.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: James Almer <jamrial@gmail.com>

doc/APIchanges

@@ -15,6 +15,9 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2018-04-01 - xxxxxxx - lavc 58.17.100 - avcodec.h
+  Add av_packet_make_refcounted().
+
 2018-xx-xx - xxxxxxx - lavfi 7.14.100 - avfilter.h
   Deprecate use of avfilter_register(), avfilter_register_all(),
   avfilter_next(). Add av_filter_iterate().