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/tree.h

@@ -19,7 +19,7 @@
  */
 
 /**
- * @file tree.h
+ * @file libavutil/tree.h
  * A tree container.
  * Insertion, removal, finding equal, largest which is smaller than and
  * smallest which is larger than, all have O(log n) worst case complexity.