x86/kvm: Fix -Wmissing-prototypes warnings

x86/kvm: Fix -Wmissing-prototypes warnings

We get two warning when build kernel with W=1:
arch/x86/kernel/kvm.c:872:6: warning: no previous prototype for ‘arch_haltpoll_enable’ [-Wmissing-prototypes]
arch/x86/kernel/kvm.c:885:6: warning: no previous prototype for ‘arch_haltpoll_disable’ [-Wmissing-prototypes]

Including the missing head file can fix this.

Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>