Remove explicit filename from Doxygen @file commands.

Passing an explicit filename to this command is only necessary if the
documentation in the @file block refers to a file different from the
one the block resides in.

Originally committed as revision 22921 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/vsrc_nullsrc.c

@@ -17,7 +17,7 @@
  */
 
 /**
- * @file libavfilter/vsrc_nullsrc.c
+ * @file
  * null video source
  */