.mailmap: add entries for Rebecca Cran

... for git-shortlog purposes.

NOTE: this patch does not introduce a cross-domain mapping; it only maps
both email addresses of Rebecca to the full name "Rebecca Cran".

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Rebecca Cran <rebecca@bsdio.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Rebecca Cran <rebecca@bsdio.com>