kernel/crash_core.c: make __crash_hotplug_lock static

sparse warnings:
kernel/crash_core.c:749:1: sparse: sparse: symbol '__crash_hotplug_lock' was not declared. Should it be static?

Fixes: e2a8f20d ("Crash: add lock to serialize crash hotplug handling")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401080654.IjjU5oK7-lkp@intel.com/
Cc: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/crash_core.c

@@ -877,7 +877,7 @@ subsys_initcall(crash_notes_memory_init);
  * regions are online. So mutex lock  __crash_hotplug_lock is used to
  * serialize the crash hotplug handling specifically.
  */
-DEFINE_MUTEX(__crash_hotplug_lock);
+static DEFINE_MUTEX(__crash_hotplug_lock);
 #define crash_hotplug_lock() mutex_lock(&__crash_hotplug_lock)
 #define crash_hotplug_unlock() mutex_unlock(&__crash_hotplug_lock)