Add an APIchanges entry for avformat_free_context

Signed-off-by: NRonald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit ff197489)

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2009-03-08
 
 API changes, most recent first:
 
+2011-02-04 - f124b08 - lavf 52.96.0 - avformat_free_context()
+  Add avformat_free_context() in avformat.h.
+
 2011-02-03 - f5b82f4 - lavc 52.109.0 - add CODEC_ID_PRORES
   Add CODEC_ID_PRORES to avcodec.h.