C: Add Brian Gitonga Marete's Google email address (Google CLA).

Change-Id: I51edc77ad4621ad8f3908e69dcb7b4eab086b5fe
Reviewed-on: https://go-review.googlesource.com/15680Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

CONTRIBUTORS

@@ -148,7 +148,7 @@ Brendan O'Dea <bod@golang.org>
 Brett Cannon <bcannon@gmail.com>
 Brian Dellisanti <briandellisanti@gmail.com>
 Brian G. Merrell <bgmerrell@gmail.com>
-Brian Gitonga Marete <marete@toshnix.com> <bgmarete@gmail.com>
+Brian Gitonga Marete <marete@toshnix.com> <bgmarete@gmail.com> <bgm@google.com>
 Brian Ketelsen <bketelsen@gmail.com>
 Brian Slesinsky <skybrian@google.com>
 Brian Smith <ohohvi@gmail.com>