BaseTools/PatchCheck.py: add exception for diff orderfile

BaseTools/PatchCheck.py: add exception for diff orderfile

SetupGit.py adds BaseTools/Conf/diff.order as a diff orderfile, but that
file currently has CRLF line endings, which causes all pattern matches
to fail and the ordering remaining unaffected.

Add an exception to PatchCheck.py (to the existing .gitmodules clause),
so that we can merge the fix to the config file.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>