powerpc: Export kvm_guest static key, for bcachefs six locks

bcachefs's six locks need kvm_guest, via
 ower_on_cpu() ->  vcpu_is_preempted() -> is_kvm_guest()

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)