powerpc: Further compile fixup for STRICT_MM_TYPECHECKS

A patch of mine was recently committed to fix up STRICT_MM_TYPECHECKS
behaviour on powerpc (f5ea64dc

).
However, something which breaks it again seems to have slipped in
afterwards.  So, here's another small fix.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>