checkpatch: report missing spaces around trigraphs with --strict

Spaces around trigraphs are specified by CodingStyle but checkpatch is
currently silent about them because there are many current instances
without them.

Make missing spaces around trigraphs a --strict message.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>