KVM: x86: fix build with !CONFIG_SMP

<asm/apic.h> isn't included directly and without CONFIG_SMP, an option
that automagically pulls it can't be enabled.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>