patman: Add a call to get_maintainer.pl if it exists

For Linux the best way to figure out where to send a patch is with the
"get_maintainer.pl" script.  Add support for calling it from patman.
Support is added unconditionally for "scripts/get_maintainer.pl" in
case it is helpful for any other projects.

Signed-off-by: Doug Anderson <dianders@chromium.org>