Forbid the items shot by siege engines if the forbid ammo order is set.

This only applies when targeting the engine, so you could still make
cheaty infinite ammo training units by using stock aiming mode.

plugins/siege-engine.cpp

@@ -1619,6 +1619,10 @@ struct projectile_hook : df::proj_itemst {
                 aimAtArea(engine, skill);
         }
 
+        bool forbid_ammo = DF_GLOBAL_VALUE(standing_orders_forbid_used_ammo, false);
+        if (forbid_ammo)
+            item->flags.bits.forbid = true;
+
         switch (item->getType())
         {
             case item_type::CAGE: