platform/surface: aggregator: Add platform-drivers-x86 list to MAINTAINERS entry

The Surface System Aggregator Module driver entry is currently missing a
mailing list. Surface platform drivers are discussed on the
platform-driver-x86 list and all other Surface platform drivers have a
reference to that list in their entries. So let's add one here as well.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210514221954.5976-1-luzmaximilian@gmail.comSigned-off-by: Hans de Goede <hdegoede@redhat.com>

MAINTAINERS

@@ -12180,6 +12180,7 @@ F:	drivers/platform/surface/surfacepro3_button.c
 
 MICROSOFT SURFACE SYSTEM AGGREGATOR SUBSYSTEM
 M:	Maximilian Luz <luzmaximilian@gmail.com>
+L:	platform-driver-x86@vger.kernel.org
 S:	Maintained
 W:	https://github.com/linux-surface/surface-aggregator-module
 C:	irc://chat.freenode.net/##linux-surface