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/libdiracenc.c

@@ -21,7 +21,7 @@
  */
 
 /**
-* @file libdiracenc.c
+* @file libavcodec/libdiracenc.c
 * Dirac encoding support via libdirac library; more details about the
 * Dirac project can be found at http://dirac.sourceforge.net/.
 * The libdirac_encoder library implements Dirac specification version 2.2