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>
2 files changed