misc: Add strndup implementation if not provides by the OS

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit 247983e982)

include/libssh/priv.h

@@ -44,6 +44,10 @@
 # endif
 #endif /* !defined(HAVE_STRTOULL) */
 
+#if !defined(HAVE_STRNDUP)
+char *strndup(const char *s, size_t n);
+#endif /* ! HAVE_STRNDUP */
+
 #ifdef HAVE_BYTESWAP_H
 #include <byteswap.h>
 #endif