lavc: postpone the removal of request_channels API.

Its replacement is still not ready.

libavcodec/version.h

@@ -47,7 +47,7 @@
  */
 
 #ifndef FF_API_REQUEST_CHANNELS
-#define FF_API_REQUEST_CHANNELS (LIBAVCODEC_VERSION_MAJOR < 55)
+#define FF_API_REQUEST_CHANNELS (LIBAVCODEC_VERSION_MAJOR < 56)
 #endif
 #ifndef FF_API_OLD_DECODE_AUDIO
 #define FF_API_OLD_DECODE_AUDIO (LIBAVCODEC_VERSION_MAJOR < 55)