KVM: PPC: Guard doorbell exception with CONFIG_PPC_DOORBELL

Availablity of the doorbell_exception function is guarded by
CONFIG_PPC_DOORBELL. Use the same define to guard our caller
of it.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
[agraf: improve patch description]
Signed-off-by: Alexander Graf <agraf@suse.de>