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: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>