log.h: make AVClass a named struct

'struct AVClass' is used in the code since
641c7afe3c17334b81e3e2eef88f1751eb68f89f, but AVClass is typedeffed as
an anonymous struct.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/log.h

@@ -30,7 +30,7 @@
  * arbitrary struct of which the first field is a pointer to an
  * AVClass struct (e.g. AVCodecContext, AVFormatContext etc.).
  */
-typedef struct {
+typedef struct AVClass {
     /**
      * The name of the class; usually it is the same name as the
      * context structure type to which the AVClass is associated.