powerpc/pseries: Fix SMP=n build of rng.c

In commit a489043f

 "Implement arch_get_random_long() based on H_RANDOM" I
broke the SMP=n build. We were getting plpar_wrappers.h via spinlock.h
which breaks when SMP=n.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>