.mailmap: add an alias for Craig Stout

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 69335e2..4e55df0 100644
--- a/.mailmap
+++ b/.mailmap
@@ -158,6 +158,8 @@
 
 Courtney Goeltzenleuchter <courtney@lunarg.com> <courtney@LunarG.com>
 
+Craig Stout <cstout@google.com>
+
 Daniel Skinner <sio@users.sourceforge.net> sio <sio>
 
 Daniel Stone <daniels@collabora.com> <daniel@fooishbar.org>