strndup() is part of XSI from The Open Group Base Specification Issue 7.

Index: string.h
===================================================================
RCS file: /cvsroot/src/include/string.h,v
retrieving revision 1.49
diff -u -p -r1.49 string.h
--- string.h	24 Sep 2014 18:16:36 -0000	1.49
+++ string.h	7 Mar 2016 19:26:38 -0000
@@ -83,6 +83,7 @@ char	*strdup(const char *);
     defined(_NETBSD_SOURCE)
 char	*stpcpy(char * __restrict, const char * __restrict);
 char	*stpncpy(char * __restrict, const char * __restrict, size_t);
+char	*strndup(const char *, size_t);
 size_t	strnlen(const char *, size_t);
 #ifndef __STRSIGNAL_DECLARED
 #define __STRSIGNAL_DECLARED
@@ -101,7 +102,6 @@ size_t	 strlcat(char *, const char *, si
 size_t	 strlcpy(char *, const char *, size_t);
 char	*strsep(char **, const char *);
 char	*stresep(char **, const char *, int);
-char	*strndup(const char *, size_t);
 char	*strnstr(const char *, const char *, size_t);
 void	*memrchr(const void *, int, size_t);
 void	*explicit_memset(void *, int, size_t);