C: add Robert Sesek (Google CLA)

LGTM=bradfitz
R=dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/61310043

CONTRIBUTORS

@@ -444,6 +444,7 @@ Robert Figueiredo <robfig@gmail.com>
 Robert Griesemer <gri@golang.org>
 Robert Hencke <robert.hencke@gmail.com>
 Robert Obryk <robryk@gmail.com>
+Robert Sesek <rsesek@google.com>
 Robert Snedegar <roberts@google.com>
 Robin Eklind <r.eklind.87@gmail.com>
 Rodrigo Moraes de Oliveira <rodrigo.moraes@gmail.com>