doc/APIchanges: fill in missing dates and hashes.

doc/APIchanges

@@ -12,19 +12,19 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
-2012-xx-xx - xxxxxxx - lavu 51.24.0 - error.h
+2012-02-25 - c9bca80 - lavu 51.24.0 - error.h
   Add AVERROR_UNKNOWN
 
-2012-xx-xx - xxxxxxx - lavc 54.x.x
+2012-02-20 - e9cda85 - lavc 54.2.0
   Add duration field to AVCodecParserContext
 
-2012-02-xx - xxxxxxx - lavu 51.23.1 - mathematics.h
+2012-02-20 - 0b42a93 - lavu 51.23.1 - mathematics.h
   Add av_rescale_q_rnd()
 
-2012-02-xx - xxxxxxx - lavu 51.22.1 - pixdesc.h
+2012-02-08 - 38d5533 - lavu 51.22.1 - pixdesc.h
   Add PIX_FMT_PSEUDOPAL flag.
 
-2012-02-01 - xxxxxxx - lavc 54.01.0
+2012-02-08 - 52f82a1 - lavc 54.01.0
   Add avcodec_encode_video2() and deprecate avcodec_encode_video().
 
 2012-02-01 - 316fc74 - lavc 54.01.0