ztest: Fix building when CONFIG_MP_NUM_CPUS=1

We get compiler warnings on testcases that set CONFIG_MP_NUM_CPUS=1
and platforms that have CONFIG_SMP=y.  Qualify the code so its only
built if CONFIG_SMP && (CONFIG_MP_NUM_CPUS > 1).

Signed-off-by: Kumar Gala <kumar.gala@intel.com>