get_maintainer: fix perl 5.22/5.24 deprecated/incompatible "\C" use

Perl 5.22 emits a deprecated message when "\C" is used in a regex.  Perl
5.24 will disallow it altogether.

Fix it by using [A-Z] instead of \C.

Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Tested-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>