lavfi: bump minor and update APIchanges after recent updates

These changes were forgotten in commits:
  lavfi: simplify signature for avfilter_get_audio_buffer() and friends
  (commit 2c0317419b)
and
  lavfi: consistently use int for sample_rate in AVFilterLink and AVFilterBufferRefAudioProps
  (commit 4381bddc9f)

doc/APIchanges

@@ -13,7 +13,10 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
-2011-09-xx - xxxxxxx - lavfi 2.x.0
+2011-09-12 - xxxxxxx - lavfi 2.40.0
+  Change AVFilterBufferRefAudioProps.sample_rate type from uint32_t to int.
+
+2011-09-12 - xxxxxxx - lavfi 2.40.0
   Simplify signature for avfilter_get_audio_buffer(), make it
   consistent with avfilter_get_video_buffer().
 

libavfilter/avfilter.h

@@ -29,7 +29,7 @@
 #include "libavutil/rational.h"
 
 #define LIBAVFILTER_VERSION_MAJOR  2
-#define LIBAVFILTER_VERSION_MINOR 39
+#define LIBAVFILTER_VERSION_MINOR 40
 #define LIBAVFILTER_VERSION_MICRO  0
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \