[POWERPC] Fix compile error with kexec and CONFIG_SMP=n

Commit 2f4dfe20 moved the definition
of hard_smp_processor_id() for the UP case from include/linux/smp.h
to include/asm/smp.h.  However, include/linux/smp.h only includes
include/asm/smp.h in the SMP case, so code that wants to use
hard_smp_processor_id() has to include <asm/smp.h> explicitly to
be sure of getting the definition.

Signed-off-by: Paul Mackerras <paulus@samba.org>