.mailmap: add an alias for Jan Beich

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index d423c8d..6e57ad8 100644
--- a/.mailmap
+++ b/.mailmap
@@ -268,6 +268,8 @@
 
 James Zhu <James.Zhu@amd.com>
 
+Jan Beich <jbeich@freebsd.org> <jbeich@FreeBSD.org>
+
 Jan Vesely <jano.vesely@gmail.com> Jan Vesely <jan.vesely@rutgers.edu>
 
 Jason Ekstrand <jason@jlekstrand.net> <jason.ekstrand@intel.com>