sparse: fix bt_lock should be static
Fix this sparse warning:
core/stack.c:123:13: warning: symbol 'bt_lock' was not declared. Should it be static?
@@ -120,7 +120,7 @@ void backtrace_print(struct bt_entry *entries, struct bt_metadata *metadata,
* a backtrace they garble each other. To prevent this we use a seperate
* lock to serialise printing of the dumps.
*/
-struct lock bt_lock = LOCK_UNLOCKED;
+static struct lock bt_lock = LOCK_UNLOCKED;
void backtrace(void)
{
