mailmap: Add entry for <rjw@sisk.pl>

The old email address I contributed to the kernel from in the past
has not been functional for a few years, so add a .mailmap entry
for it.

Signed-off-by: Rafael J. Wysocki <rjw@rjwysocki.net>

.mailmap

@@ -215,6 +215,7 @@ Praveen BP <praveenbp@ti.com>
 Punit Agrawal <punitagrawal@gmail.com> <punit.agrawal@arm.com>
 Qais Yousef <qsyousef@gmail.com> <qais.yousef@imgtec.com>
 Quentin Perret <qperret@qperret.net> <quentin.perret@arm.com>
+Rafael J. Wysocki <rjw@rjwysocki.net> <rjw@sisk.pl>
 Rajesh Shah <rajesh.shah@intel.com>
 Ralf Baechle <ralf@linux-mips.org>
 Ralf Wildenhues <Ralf.Wildenhues@gmx.de>