checkpatch: Escape left braces in regex

Latest perl now deprecates "{" literal in regex and print warnings like
"unescaped left brace in regex is deprecated".  Add escape to keep it
happy.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1441969656-2640-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>