sched/core: Make sched_dynamic_mutex static

The sched_dynamic_mutex is only used within the file. Make it static.

Fixes: e3ff7c609f39 ("livepatch,sched: Add livepatch task switching to cond_resched()")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/oe-kbuild-all/[e-mail]/

kernel/sched/core.c

···
 #error "Unsupported PREEMPT_DYNAMIC mechanism"
 #endif
 
-DEFINE_MUTEX(sched_dynamic_mutex);
+static DEFINE_MUTEX(sched_dynamic_mutex);
 static bool klp_override;
 
 static void __sched_dynamic_update(int mode)