KVM: arm/arm64: vgic: Remove useless arm-gic.h #include

Back in the days, vgic.c used to have an intimate knowledge of
the actual GICv2. These days, this has been abstracted away into
hardware-specific backends.

Remove the now useless arm-gic.h #include directive, making it
clear that GICv2 specific code doesn't belong here.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>