lavc: add a wrapper for AVCodecContext.get_buffer().

It will be useful in the upcoming transition to refcounted AVFrames.
diff --git a/libavcodec/cdxl.c b/libavcodec/cdxl.c
index bc4dfe7..b51a2cb 100644
--- a/libavcodec/cdxl.c
+++ b/libavcodec/cdxl.c
@@ -23,6 +23,7 @@
 #include "libavutil/imgutils.h"
 #include "avcodec.h"
 #include "get_bits.h"
+#include "internal.h"
 
 #define BIT_PLANAR   0x00
 #define BYTE_PLANAR  0x20
@@ -262,7 +263,7 @@
         avctx->release_buffer(avctx, p);
 
     p->reference = 0;
-    if ((ret = avctx->get_buffer(avctx, p)) < 0) {
+    if ((ret = ff_get_buffer(avctx, p)) < 0) {
         av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
         return ret;
     }