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/mlp_parser.c

@@ -20,7 +20,7 @@
  */
 
 /**
- * @file mlp_parser.c
+ * @file libavcodec/mlp_parser.c
  * MLP parser
  */