checkpatch: Don't complain about MIME headers.

When patches contain extended character sets, patches will contain
MIME headers after the subject line, which should not be confused
for a too-long summary line.

Change-Id: I66f70b465ef804444a3bdb49867654b5613b13a7
Signed-off-by: Gregory Bean <gbean@codeaurora.org>
(cherry picked from commit 8e6b9d3790595198a34320f1c3f4504cd258fed1)
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>