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/wmadec.c

@@ -20,7 +20,7 @@
  */
 
 /**
- * @file wmadec.c
+ * @file libavcodec/wmadec.c
  * WMA compatible decoder.
  * This decoder handles Microsoft Windows Media Audio data, versions 1 & 2.
  * WMA v1 is identified by audio format 0x160 in Microsoft media files