uvm: Make uvm_extern.h (more) self-contained, needs sys/types.h.

sys/uvm/uvm_extern.h

@@ -1,4 +1,4 @@
-/*	$NetBSD: uvm_extern.h,v 1.231 2020/08/14 09:06:15 chs Exp $	*/
+/*	$NetBSD: uvm_extern.h,v 1.232 2021/05/31 10:57:02 riastradh Exp $	*/
 
 /*
  * Copyright (c) 1997 Charles D. Cranor and Washington University.
@@ -72,6 +72,8 @@
  * NOTE: vm system calls are prototyped in syscallargs.h
  */
 
+#include <sys/types.h>
+
 /*
  * defines
  */