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>

include/linux/string.h

@@ -93,8 +93,8 @@ size_t strcspn(const char *s, const char *reject);
 
 #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