KVM: VMX: Make vmx_load_mmu_pgd() static

Make vmx_load_mmu_pgd() static as it is no longer invoked directly by
nested VMX (or any code for that matter).

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200716034122.5998-5-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>