[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>

arch/powerpc/platforms/pseries/kexec.c

@@ -12,6 +12,7 @@
 #include <asm/firmware.h>
 #include <asm/kexec.h>
 #include <asm/mpic.h>
+#include <asm/smp.h>
 
 #include "pseries.h"
 #include "xics.h"