xen/arm: move pending_irq structure to vgic header file

xen/arm: move pending_irq structure to vgic header file

gic.h requires definition of pending_irq structure defined
in domain.h and domain.h requires gic_state structure defined
in gic.h and hence there is inter-dependency between domain.h
and gic.h files.

So move pending_irq to vgic.h which is relevant place for this
structure and break domain.h and gic.h interdependency

By this move irq_to_pending function declaration from gic.h
to vgic.h

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>