hotplug: Fix UP bug in smpboot hotplug code

Because kernel subsystems need their per-CPU kthreads on UP systems as
well as on SMP systems, the smpboot hotplug kthread functions must be
provided in UP builds as well as in SMP builds.  This commit therefore
adds smpboot.c to UP builds and excludes irrelevant code via #ifdef.
    
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>