Compiling BL31 for the Allwinner platform now produces a message about
the deprecation of gic_common.c.
Follow the advice and use include gicv2.mk instead.

Collect all includes at the beginning of the file on the way.

Change-Id: Iee46e21a630bfa831d28059f09aa7b049eb554bb
Signed-off-by: Andre Przywara <andre.przywara@arm.com>