kvm: x86: make a function in x86.c static to avoid compiling warning

kvm_emulate_wbinvd_noskip is only used in x86.c, and should be
static to avoid compiling warning when with -Wmissing-prototypes
option.
Signed-off-by: NJiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -4795,7 +4795,7 @@ static void emulator_invlpg(struct x86_emulate_ctxt *ctxt, ulong address)
 	kvm_mmu_invlpg(emul_to_vcpu(ctxt), address);
 }
 
-int kvm_emulate_wbinvd_noskip(struct kvm_vcpu *vcpu)
+static int kvm_emulate_wbinvd_noskip(struct kvm_vcpu *vcpu)
 {
 	if (!need_emulate_wbinvd(vcpu))
 		return X86EMUL_CONTINUE;