sched/uclamp: Fix unnused variable warning

uclamp_mutex is only used for CONFIG_SYSCTL or
CONFIG_UCLAMP_TASK_GROUP so declare it __maybe_unused.

Closes: https://lore.kernel.org/oe-kbuild-all/202410060258.bPl2ZoUo-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202410250459.EJe6PJI5-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/a1e9c342-01c9-44f0-a789-2c908e57942b@arm.com

kernel/sched/core.c

@@ -1399,7 +1399,7 @@ void set_load_weight(struct task_struct *p, bool update_load)
  * requests are serialized using a mutex to reduce the risk of conflicting
  * updates or API abuses.
  */
-static DEFINE_MUTEX(uclamp_mutex);
+static __maybe_unused DEFINE_MUTEX(uclamp_mutex);
 
 /* Max allowed minimum utilization */
 static unsigned int __maybe_unused sysctl_sched_uclamp_util_min = SCHED_CAPACITY_SCALE;