Add multiple inclusion protection.

PR:		165025
Submitted by:	Gavin Mu
MFC after:	1 week

sys/boot/common/bootstrap.h

@@ -26,6 +26,9 @@
  * $FreeBSD$
  */
 
+#ifndef _BOOTSTRAP_H_
+#define	_BOOTSTRAP_H_
+
 #include <sys/types.h>
 #include <sys/queue.h>
 #include <sys/linker_set.h>
@@ -323,3 +326,5 @@ void	delay(int delay);
 void	dev_cleanup(void);
 
 time_t	time(time_t *tloc);
+
+#endif /* !_BOOTSTRAP_H_ */