selftests/sched_ext: Fix build error in dequeue selftest

Building the dequeue selftest with newer compilers (e.g., gcc 16)
triggers the following error:

 dequeue.c:28:22: error: variable 'sum' set but not used

The 'volatile' qualifier prevents the writes from being optimized away,
but does not silence the unused variable 'sum' is indeed only written
and never read.

Consume 'sum' via an empty asm() with a register input constraint. This
forces the compiler to keep the accumulated value (preserving the CPU
stress loop) and avoiding the build error.

Fixes: 658ad2259b ("selftests/sched_ext: Add test to validate ops.dequeue() semantics")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/sched_ext/dequeue.c

@@ -33,6 +33,7 @@ static void worker_fn(int id)
 		/* Do some work to trigger scheduling events */
 		for (j = 0; j < 10000; j++)
 			sum += j;
+		asm volatile("" : : "r"(sum));
 
 		/* Sleep to trigger dequeue */
 		usleep(1000 + (id * 100));