[PATCH] powerpc: Fix !SMP build of rtas.c

arch/powerpc/kernel/rtas.c is getting hvcall.h via spinlock.h, but when we're
building for UP we don't include spinlock.h.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>