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/include/linux/string.h b/include/linux/string.h
index 5d63be4..bb1d5ab 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -93,8 +93,8 @@
 
 #ifndef __HAVE_ARCH_STRDUP
 extern char * strdup(const char *);
-#endif
 extern char * strndup(const char *, size_t);
+#endif
 #ifndef __HAVE_ARCH_STRSWAB
 extern char * strswab(const char *);
 #endif
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
 /**