sched/core: Make sched_dynamic_mutex static

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

Fixes: e3ff7c60 ("livepatch,sched: Add livepatch task switching to cond_resched()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/oe-kbuild-all/202304062335.tNuUjgsl-lkp@intel.com/