kvm: x86: hyperv: make function static to avoid compiling warning

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

arch/x86/kvm/hyperv.c

@@ -291,7 +291,7 @@ static int synic_get_msr(struct kvm_vcpu_hv_synic *synic, u32 msr, u64 *pdata)
 	return ret;
 }
 
-int synic_set_irq(struct kvm_vcpu_hv_synic *synic, u32 sint)
+static int synic_set_irq(struct kvm_vcpu_hv_synic *synic, u32 sint)
 {
 	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
 	struct kvm_lapic_irq irq;