sched: Disable -Wunused-but-set-variable

The compilers can't deal with obvious DCE vs that warning, resulting
in code like:

	if (0) {
		sched sched_statistics *stats;

		stats = __schedstats_from_se(se);

		...
	}

triggering the warning. Kill the warning to make the robots stop
reporting this.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lkml.kernel.org/r/YWWPLnaZGybHsTkv@hirez.programming.kicks-ass.net