make sure there is a prototype in scope.

lib/libc/arch/arm/gen/fabs.c

@@ -1,4 +1,4 @@
-/*	$NetBSD: fabs.c,v 1.2 2002/05/26 11:48:01 wiz Exp $	*/
+/*	$NetBSD: fabs.c,v 1.3 2006/05/16 05:29:16 mrg Exp $	*/
 
 /*
  * Copyright (c) 1996 Mark Brinicombe
@@ -35,6 +35,8 @@
  * fabs(x) returns the absolute value of x.
  */
 
+double fabs(double x);
+
 double
 fabs(double x)
 {