x86: build fix

Impact: build fix on certain UP configs

fix:

 arch/x86/kernel/cpu/common.c: In function 'cpu_init':
 arch/x86/kernel/cpu/common.c:1141: error: 'boot_cpu_id' undeclared (first use in this function)
 arch/x86/kernel/cpu/common.c:1141: error: (Each undeclared identifier is reported only once
 arch/x86/kernel/cpu/common.c:1141: error: for each function it appears in.)

Pull in asm/smp.h on UP, so that we get the definition of
boot_cpu_id.

Signed-off-by: Ingo Molnar <mingo@elte.hu>