KVM: arm/arm64: vgic-its: Make attribute accessors static

Fix sparse warnings:

arch/arm64/kvm/../../../virt/kvm/arm/vgic/vgic-its.c:1732:5: warning:
 symbol 'vgic_its_has_attr_regs' was not declared. Should it be static?
arch/arm64/kvm/../../../virt/kvm/arm/vgic/vgic-its.c:1753:5: warning:
 symbol 'vgic_its_attr_regs_access' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
[maz: fixed subject]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>