C: add Tobias Columbus's email used in Rietveld

R=golang-dev, tobias.columbus, bradfitz
CC=golang-dev
https://golang.org/cl/6483058

CONTRIBUTORS

@@ -308,7 +308,7 @@ Tarmigan Casebolt <tarmigan@gmail.com>
 Taru Karttunen <taruti@taruti.net>
 Thomas Alan Copeland <talan.copeland@gmail.com>
 Timo Savola <timo.savola@gmail.com>
-Tobias Columbus <tobias.columbus@gmail.com>
+Tobias Columbus <tobias.columbus@gmail.com> <tobias.columbus@googlemail.com>
 Tom Szymanski <tgs@google.com>
 Tor Andersson <tor.andersson@gmail.com>
 Trevor Strohman <trevor.strohman@gmail.com>