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/fifo.h

@@ -17,7 +17,7 @@
  */
 
 /**
- * @file fifo.h
+ * @file libavutil/fifo.h
  * a very simple circular buffer FIFO implementation
  */