.mailmap: add an alias for Xavier Bouchoux

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 4a5787e..681459f 100644
--- a/.mailmap
+++ b/.mailmap
@@ -621,6 +621,8 @@
 
 Wladimir J. van der Laan <laanwj@gmail.com>
 
+Xavier Bouchoux <xavierb@gmail.com>
+
 Zhenyu Wang <zhenyuw@linux.intel.com> Wang Zhenyu <zhenyu.z.wang@intel.com>
 
 Zack Rusin <zackr@vmware.com> <zack@kde.org>