powerpc: Fix build errors with UP configs in HV-style KVM

This fixes these errors when building UP with CONFIG_KVM_BOOK3S_64_HV=y:

arch/powerpc/kvm/book3s_hv.c:1855:2: error: implicit declaration of function 'inhibit_secondary_onlining' [-Werror=implicit-function-declaration]
arch/powerpc/kvm/book3s_hv.c:1862:2: error: implicit declaration of function 'uninhibit_secondary_onlining' [-Werror=implicit-function-declaration]
cc1: all warnings being treated as errors

and this error (with CONFIG_KVM_BOOK3S_64=m, or a vmlinux link error
with CONFIG_KVM_BOOK3S_64=y):

ERROR: "smp_send_reschedule" [arch/powerpc/kvm/kvm.ko] undefined!
make[2]: *** [__modpost] Error 1

The fix for the link error is suboptimal; ideally we want a self_ipi()
function from irq.c, connected at least to the MPIC code, to initiate
an IPI to this cpu.  The fix here at least lets the code build, and it
will work, just with interrupts being delayed sometimes.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>