Changelog: Add 4.3 cut marker

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
-version <next>:
+version 4.3:
 - v360 filter
 - Intel QSV-accelerated MJPEG decoding
 - Intel QSV-accelerated VP9 decoding