x86: kvm: mmu: make kvm_mmu_clear_all_pte_masks static

x86: kvm: mmu: make kvm_mmu_clear_all_pte_masks static

The kvm_mmu_clear_all_pte_masks interface is only used by kvm_mmu_module_init
locally, and does not need to be called by other module, make it static.

This patch cleans up sparse warning:
symbol 'kvm_mmu_clear_all_pte_masks' was not declared. Should it be static?

Signed-off-by: Gimcuan Hui <gimcuan@gmail.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>