.mailmap: add an alias for Tim Wiederhake

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1999>
diff --git a/.mailmap b/.mailmap
index 5b8fd66..94a05fb 100644
--- a/.mailmap
+++ b/.mailmap
@@ -578,6 +578,8 @@
 
 Tilman Sauerbeck <tilman@code-monkey.de> <tilman@freedesktop.org>
 
+Tim Wiederhake <twied@gmx.net>
+
 Timothy Arceri <timothy.arceri@collabora.com> <t_arceri@yahoo.com.au>
 Timothy Arceri <timothy.arceri@collabora.com> Timothy <t_arceri@yahoo.com.au>