.mailmap: add an alias for Christian Gmeiner

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index b83d9ba..1c3facb 100644
--- a/.mailmap
+++ b/.mailmap
@@ -124,6 +124,8 @@
 
 Chih-Wei Huang <cwhuang@linux.org.tw> Chih-Wei Huang <cwhuang@android-x86.org>
 
+Christian Gmeiner <christian.gmeiner@gmail.com> <christian.GMEINER@bachmann.info>
+
 Christian König <christian.koenig@amd.com> Christian Koenig <christian.koenig@amd.com>
 Christian König <christian.koenig@amd.com> Christian König <christian.koenig at amd.com>
 Christian König <christian.koenig@amd.com> Christian König <deathsimple@vodafone.de>