kmemleak: Allow kmemleak to be built on powerpc

Very lightly tested, doesn't crash the kernel.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>