Use full internal pathname in doxygen @file directives.

Otherwise doxygen complains about ambiguous filenames when files exist
under the same name in different subdirectories.

Originally committed as revision 16912 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo_common.h

@@ -23,7 +23,7 @@
  */
 
 /**
- * @file mpegvideo_common.h
+ * @file libavcodec/mpegvideo_common.h
  * The simplest mpeg encoder (well, it was the simplest!).
  */