hw/arm/virt: ARM_VIRT must select ARM_GIC

The removal of the selection of A15MPCORE from ARM_VIRT also
removed what A15MPCORE selects, ARM_GIC. We still need ARM_GIC.

Fixes: bec3c97e ("hw/arm/virt: Remove dependency on Cortex-A15 MPCore peripherals")
Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201111143440.112763-1-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>