avio: make AVIOContext.av_class pointer to const

Fix this warning:
libavformat/aviobuf.c:663:20: warning: assignment discards qualifiers from pointer target type

Although this is a public header, it should remain source and
binary compatible.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/avio.h

@@ -78,7 +78,7 @@ typedef struct {
      * warning -- this field can be NULL, be sure to not pass this AVIOContext
      * to any av_opt_* functions in that case.
      */
-    AVClass *av_class;
+    const AVClass *av_class;
     unsigned char *buffer;  /**< Start of the buffer. */
     int buffer_size;        /**< Maximum buffer size */
     unsigned char *buf_ptr; /**< Current position in the buffer */