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/vp3dsp.c

@@ -19,7 +19,7 @@
  */
 
 /**
- * @file vp3dsp.c
+ * @file libavcodec/vp3dsp.c
  * Standard C DSP-oriented functions cribbed from the original VP3
  * source code.
  */