lavu: bump lavu minor for av_popcount64

Add it to doc/APIchanges and update and correct other entries.

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
+2011-01-03 - b73ec05 - lavu 51.21.0
+  Add av_popcount64
+
 2011-12-25 - lavfi 2.14.0
   e1d9dbf Add a new installed header - buffersrc.h
   It contains a new function av_buffersrc_buffer() that allows passing
@@ -30,18 +33,18 @@ API changes, most recent first:
 2011-12-18 - 8400b12 - lavc 53.28.1
   Deprecate AVFrame.age. The field is unused.
 
-2011-xx-xx - xxxxxxx - lavf 53.17.0
-  Add avformat_open_input().
+2011-12-12 - 5266045 - lavf 53.17.0
+  Add avformat_close_input().
   Deprecate av_close_input_file() and av_close_input_stream().
 
-2011-xx-xx - xxxxxxx - lavc 53.25.0
+2011-12-02 - 0eea212 - lavc 53.25.0
   Add nb_samples and extended_data fields to AVFrame.
   Deprecate AVCODEC_MAX_AUDIO_FRAME_SIZE.
   Deprecate avcodec_decode_audio3() in favor of avcodec_decode_audio4().
   avcodec_decode_audio4() writes output samples to an AVFrame, which allows
   audio decoders to use get_buffer().
 
-2011-xx-xx - xxxxxxx - lavc 53.24.0
+2011-12-04 - 560f773 - lavc 53.24.0
   Change AVFrame.data[4]/base[4]/linesize[4]/error[4] to [8] at next major bump.
   Change AVPicture.data[4]/linesize[4] to [8] at next major bump.
   Change AVCodecContext.error[4] to [8] at next major bump.

libavutil/avutil.h

@@ -154,7 +154,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 51
-#define LIBAVUTIL_VERSION_MINOR 20
+#define LIBAVUTIL_VERSION_MINOR 21
 #define LIBAVUTIL_VERSION_MICRO  0
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \