sched: CONFIG_SCHED_GROUP_FAIR=y fixlet

when I built with CONFIG_FAIR_GROUP_SCHED=y, I need the following change
to make things right.

[ From: mingo@elte.hu ]

this config option is not upstream-configurable right now but lets fix
this for completeness.

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>