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>