mm: sched: numa: Control enabling and disabling of NUMA balancing if !SCHED_DEBUG

The "mm: sched: numa: Control enabling and disabling of NUMA balancing"
depends on scheduling debug being enabled but it's perfectly legimate to
disable automatic NUMA balancing even without this option. This should
take care of it.

Signed-off-by: Mel Gorman <mgorman@suse.de>