KVM: PPC: Move kvmhv_on_pseries() into kvm_ppc.h

We recently introduced a usage of kvmhv_on_pseries() in powerpc.c, which
causes a build error for ppc64_book3e_allmodconfig:

  arch/powerpc/kvm/powerpc.c:716:8: error: implicit declaration of function ‘kvmhv_on_pseries’
    716 |    if (kvmhv_on_pseries()) {
        |        ^~~~~~~~~~~~~~~~

Fix it by moving kvmhv_on_pseries() into kvm_ppc.h so that the stub
version is available for book3e builds.

Fixes: f771b557 ("KVM: PPC: Use KVM_CAP_PPC_AIL_MODE_3")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>