dontdiff: ignore timeconst.h

Ignore the autobuilt kernel/timeconst.h when
using diff on an built kernel tree.
Signed-off-by: NBen Dooks <ben-linux@fluff.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

Documentation/dontdiff

@@ -172,6 +172,7 @@ sm_tbl*
 split-include
 tags
 tftpboot.img
+timeconst.h
 times.h*
 tkparse
 trix_boot.h