make this idempotent. bus.h needs to include it, and other things which

may include something that includes bus.h may include it as well.

sys/arch/i386/include/pio.h

@@ -1,4 +1,4 @@
-/*	$NetBSD: pio.h,v 1.12 1995/12/20 07:53:09 mycroft Exp $	*/
+/*	$NetBSD: pio.h,v 1.13 1996/03/08 20:15:23 cgd Exp $	*/
 
 /*
  * Copyright (c) 1993, 1995 Charles M. Hannum.  All rights reserved.
@@ -29,6 +29,9 @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifndef _I386_PIO_H_
+#define _I386_PIO_H_
+
 /*
  * Functions to provide access to i386 programmed I/O instructions.
  *
@@ -205,3 +208,5 @@ outsl(int port, void *addr, int cnt)
 			 "d" (port), "S" (addr), "c" (cnt)	:
 			 "%esi", "%ecx");
 }
+
+#endif /* _I386_PIO_H_ */