.mailmap: specify spelling for Gurchetan Singh

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 0cd2681..cc8d88a 100644
--- a/.mailmap
+++ b/.mailmap
@@ -227,6 +227,8 @@
 
 Gert Wollny <gert.wollny@collabora.com> <gw.fossdev@gmail.com>
 
+Gurchetan Singh <gurchetansingh@chromium.org>
+
 Gwenole Beauchesne <gwenole.beauchesne@intel.com> <gb.devel@gmail.com>
 
 Hamish Marson <hmarson@users.sourceforge.net> hmarson <hmarson>