sched: numa: Fix build error if CONFIG_NUMA_BALANCING && !CONFIG_TRANSPARENT_HUGEPAGE

Michal Hocko reported that the following build error occurs if
CONFIG_NUMA_BALANCING is set without THP support

  kernel/sched/fair.c: In function ‘task_numa_work’:
  kernel/sched/fair.c:932:55: error: call to ‘__build_bug_failed’ declared with attribute error: BUILD_BUG failed

The problem is that HPAGE_PMD_SHIFT triggers a BUILD_BUG() on
!CONFIG_TRANSPARENT_HUGEPAGE. This patch addresses the problem.

Reported-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>