KVM: x86: fix warning on 32-bit compilation

PCIDs are only supported in 64-bit mode.  No need to clear bit 63
of CR3 unless the host is 64-bit.

Reported by Fengguang Wu's autobuilder.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -750,7 +750,9 @@ EXPORT_SYMBOL_GPL(kvm_set_cr4);
 
 int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 {
+#ifdef CONFIG_X86_64
 	cr3 &= ~CR3_PCID_INVD;
+#endif
 
 	if (cr3 == kvm_read_cr3(vcpu) && !pdptrs_changed(vcpu)) {
 		kvm_mmu_sync_roots(vcpu);