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

libavformat/options.c

@@ -21,7 +21,7 @@
 #include "libavcodec/opt.h"
 
 /**
- * @file options.c
+ * @file libavformat/options.c
  * Options definition for AVFormatContext.
  */