Remove duplicated code.

Suggested by:	jmg

lib/libc/string/Makefile.inc

@@ -2,6 +2,7 @@
 # $FreeBSD$
 
 .PATH: ${LIBC_SRCTOP}/${LIBC_ARCH}/string ${LIBC_SRCTOP}/string
+.PATH: ${LIBC_SRCTOP}/../../sys/libkern
 
 CFLAGS+= -I${LIBC_SRCTOP}/locale
 

lib/libc/string/explicit_bzero.c

@@ -1,22 +0,0 @@
-/*	$OpenBSD: explicit_bzero.c,v 1.3 2014/06/21 02:34:26 matthew Exp $ */
-/*
- * Public domain.
- * Written by Matthew Dempsky.
- */
-
-#include <sys/cdefs.h>
-__FBSDID("$FreeBSD$");
-
-#include <string.h>
-
-__attribute__((weak)) void
-__explicit_bzero_hook(void *buf, size_t len)
-{
-}
-
-void
-explicit_bzero(void *buf, size_t len)
-{
-	memset(buf, 0, len);
-	__explicit_bzero_hook(buf, len);
-}

sys/libkern/explicit_bzero.c

@@ -7,7 +7,11 @@
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");
 
+#ifdef _KERNEL
 #include <sys/libkern.h>
+#else
+#include <string.h>
+#endif /* _KERNEL */
 
 __attribute__((weak)) void __explicit_bzero_hook(void *, size_t);