powerpc/mpic_msgr: fix compile error when SMP disabled

In file included from arch/powerpc/sysdev/mpic_msgr.c:20:0:
~/arch/powerpc/include/asm/mpic_msgr.h: In function 'mpic_msgr_set_destination':
~/arch/powerpc/include/asm/mpic_msgr.h:117:2:
error: implicit declaration of function 'get_hard_smp_processor_id'
make[1]: *** [arch/powerpc/sysdev/mpic_msgr.o] Error 1

Signed-off-by: Mingkai Hu <Mingkai.hu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>