KVM: PPC: Book3S HV: Fix compilation without CONFIG_PPC_POWERNV

Fixes this build breakage:

arch/powerpc/kvm/book3s_hv_ras.c: In function ‘kvmppc_realmode_mc_power7’:
arch/powerpc/kvm/book3s_hv_ras.c:126:23: error: ‘struct paca_struct’ has no member named ‘opal_mc_evt’

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Alexander Graf <agraf@suse.de>