sched: fix build error in kernel/sched_rt.c when RT_GROUP_SCHED && !SMP

Ingo found a build error in the scheduler when RT_GROUP_SCHED was
enabled, but SMP was not.  This patch rearranges the code such
that it is a little more streamlined and compiles under all permutations
of SMP, UP and RT_GROUP_SCHED.  It was boot tested on my 4-way x86_64
and it still passes preempt-test.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>