checkpatch: fix another left brace warning

This patch escapes a regex that uses left brace.

Using checkpatch.pl with Perl 5.22.0 generates the warning: "Unescaped
left brace in regex is deprecated, passed through in regex;"

Comment from regcomp.c in Perl source: "Currently we don't warn when the
lbrace is at the start of a construct.  This catches it in the middle of
a literal string, or when it's the first thing after something like
"\b"."

This works as a complement to 4e5d56bd ("checkpatch: fix left brace
warning").

Signed-off-by: Geyslan G. Bem <geyslan@gmail.com>
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Peter Senna Tschudin <peter.senna@gmail.com>
Cc: Eddie Kovsky <ewk@edkovsky.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>