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

libavutil/base64.c

@@ -19,7 +19,7 @@
  */
 
 /**
- * @file base64.c
+ * @file libavutil/base64.c
  * @brief Base64 encode/decode
  * @author Ryan Martell <rdm4@martellventures.com> (with lots of Michael)
  */