half-guard the actual function prototypes.

precludes -Wredundant-declarations with multiple includes.
okay millert@

src/include/assert.h

@@ -1,4 +1,4 @@
-/*	$OpenBSD: assert.h,v 1.12 2006/01/31 10:53:51 hshoexer Exp $	*/
+/*	$OpenBSD: assert.h,v 1.13 2011/06/26 21:11:41 espie Exp $	*/
 /*	$NetBSD: assert.h,v 1.6 1994/10/26 00:55:44 cgd Exp $	*/
 
 /*-
@@ -59,7 +59,10 @@
 # endif
 #endif
 
+#ifndef _ASSERT_H_
+#define _ASSERT_H_
 __BEGIN_DECLS
 __dead void __assert(const char *, int, const char *);
 __dead void __assert2(const char *, int, const char *, const char *);
 __END_DECLS
+#endif