sched/headers: Add header guard to <linux/sched/deadline.h>

It's the only non-trivial header in include/linux/sched/ missing a header guard.
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched/deadline.h

 /* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_SCHED_DEADLINE_H
+#define _LINUX_SCHED_DEADLINE_H
 
 /*
  * SCHED_DEADLINE tasks has negative priorities, reflecting
@@ -34,3 +36,5 @@ extern void dl_add_task_root_domain(struct task_struct *p);
 extern void dl_clear_root_domain(struct root_domain *rd);
 
 #endif /* CONFIG_SMP */
+
+#endif /* _LINUX_SCHED_DEADLINE_H */