cql3: expr: restrictions.hh: protect against double inclusions

Add #pragma once. Right now it's safe as it only has declarations
(which can be repeated), but soon it will have a definition.

cql3/expr/restrictions.hh

@@ -6,6 +6,8 @@
  * SPDX-License-Identifier: AGPL-3.0-or-later
  */
 
+#pragma once
+
 #include "expression.hh"
 
 namespace cql3 {