.mailmap: add an alias for Haihao Xiang

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index cc8d88a..7c967de 100644
--- a/.mailmap
+++ b/.mailmap
@@ -231,6 +231,8 @@
 
 Gwenole Beauchesne <gwenole.beauchesne@intel.com> <gb.devel@gmail.com>
 
+Haihao Xiang <haihao.xiang@intel.com>
+
 Hamish Marson <hmarson@users.sourceforge.net> hmarson <hmarson>
 
 Hans de Goede <hdegoede@redhat.com> Hans de Goede <j.w..r..degoede@hhs.nl>