C: add Rui Ueyama (Google CLA)

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/76930044

CONTRIBUTORS

@@ -476,6 +476,7 @@ Roger Peppe <rogpeppe@gmail.com>
 Ron Minnich <rminnich@gmail.com>
 Ross Light <rlight2@gmail.com>
 Rowan Worth <sqweek@gmail.com>
+Rui Ueyama <ruiu@google.com>
 Russ Cox <rsc@golang.org>
 Ryan Barrett <ryanb@google.com>
 Ryan Hitchman <hitchmanr@gmail.com>