APIChanges: add entry for libavresample

Appears this was forgotten
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/APIchanges

@@ -8,6 +8,7 @@ libavfilter:   2011-04-18
 libavformat:   2012-01-27
 libavresample: 2012-04-24
 libpostproc:   2011-04-18
+libswresample: 2011-09-19
 libswscale:    2011-06-20
 libavutil:     2011-04-18