Add strnlen() to local C library

This code has been imported and slightly adapted from FreeBSD:
https://github.com/freebsd/freebsd/blob/6253393ad8df55730481bf2aafd76bdd6182e2f5/lib/libc/string/strnlen.c

Change-Id: Ie5ef5f92e6e904adb88f8628077fdf1d27470eb3
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/lib/stdlib/stdlib.mk b/lib/stdlib/stdlib.mk
index cdf3d29..4ff405f 100644
--- a/lib/stdlib/stdlib.mk
+++ b/lib/stdlib/stdlib.mk
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+# Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are met:
@@ -41,6 +41,7 @@
 			strcmp.c			\
 			strlen.c			\
 			strncmp.c			\
+			strnlen.c			\
 			subr_prf.c)
 
 INCLUDES	+=	-Iinclude/lib/stdlib		\