Add constraint_reset() stub when libtls is not available

This prevents a linking error since ntp_dispatch_imsg calls
constraint_reset().

src/constraint-disabled.c

@@ -66,6 +66,11 @@ constraint_add(struct constraint *cstr)
 	free(cstr);
 }
 
+void
+constraint_reset(void)
+{
+}
+
 void
 constraint_msg_dns(u_int32_t id, u_int8_t *data, size_t len)
 {