headers_check fix: linux/dn.h

fix the following 'make headers_check' warning:

  usr/include/linux/dn.h:75: found __[us]{8,16,32,64} type without #include <linux/types.h>
Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>

include/linux/dn.h

 #ifndef _LINUX_DN_H
 #define _LINUX_DN_H
 
+#include <linux/types.h>
+
 /*
 
 	DECnet Data Structures and Constants