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

In commit a489043 "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>

arch/powerpc/platforms/pseries/rng.c

@@ -13,6 +13,7 @@
 #include <linux/of.h>
 #include <asm/archrandom.h>
 #include <asm/machdep.h>
+#include <asm/plpar_wrappers.h>
 
 
 static int pseries_get_random_long(unsigned long *v)