sched: fix warning in inc_rt_tasks() to not declare variable 'rq' if it's not needed

Fix inc_rt_tasks() to not declare variable 'rq' if it's not needed.  It is
declared if CONFIG_SMP or CONFIG_RT_GROUP_SCHED, but only used if CONFIG_SMP.

This is a consequence of patch 1f11eb6a plus
patch 1100ac91

.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>