mailmap: Add an entry for Axel Lin.

Not all of Axel's patches have used a consistent casing, so fix it up
here.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

.mailmap

@@ -23,6 +23,7 @@ Andy Adamson <andros@citi.umich.edu>
 Arnaud Patard <arnaud.patard@rtp-net.org>
 Arnd Bergmann <arnd@arndb.de>
 Axel Dyks <xl@xlsigned.net>
+Axel Lin <axel.lin@gmail.com>
 Ben Gardner <bgardner@wabtec.com>
 Ben M Cahill <ben.m.cahill@intel.com>
 Björn Steinbrink <B.Steinbrink@gmx.de>