kvm: x86: make function static to avoid compiling warning

vmx_arm_hv_timer is only used in vmx.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/vmx.c

@@ -8709,7 +8709,7 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
 					msrs[i].host);
 }
 
-void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
+static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	u64 tscl;