string: Allow arch override of strndup() also

At present architectures can override strdup() but not strndup(). Use
the same option for both.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/string.c b/lib/string.c
index 9b779dd..ae7835f 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -324,7 +324,6 @@
 	strcpy (new, s);
 	return new;
 }
-#endif
 
 char * strndup(const char *s, size_t n)
 {
@@ -348,6 +347,7 @@
 
 	return new;
 }
+#endif
 
 #ifndef __HAVE_ARCH_STRSPN
 /**