lavc: postpone the removal of AVCodecContext.request_channels.

Although it has been deprecated for a long time, its intended
replacement (request_channel_layout) is not actually used anywhere, so
request_channels is currently the only way to access that functionality.

libavcodec/version.h

@@ -51,7 +51,7 @@
 #define FF_API_ANTIALIAS_ALGO   (LIBAVCODEC_VERSION_MAJOR < 54)
 #endif
 #ifndef FF_API_REQUEST_CHANNELS
-#define FF_API_REQUEST_CHANNELS (LIBAVCODEC_VERSION_MAJOR < 54)
+#define FF_API_REQUEST_CHANNELS (LIBAVCODEC_VERSION_MAJOR < 55)
 #endif
 #ifndef FF_API_OPT_H
 #define FF_API_OPT_H            (LIBAVCODEC_VERSION_MAJOR < 54)