x86 boot: include missing smp.h header

The patch:
    x86: convert cpu_to_apicid to be a per cpu variable
introduced a dependency of ipi.h on smp.h in x86
builds with an allnoconfig.  Including smp.h in ipi.h
fixes the build error:
    In file included from arch/x86/kernel/traps_64.c:48:
    include/asm/ipi.h: In function 'send_IPI_mask_sequence':
    include/asm/ipi.h:114: error: 'per_cpu__x86_cpu_to_apicid' undeclared (first use in this function)

Signed-off-by: Paul Jackson <pj@sgi.com>
Cc: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>