recruit_rushers CA: check if castle_switch CA is present

So that it is possible to use the recruit_rushers CA without the castle_switch CA.

data/ai/lua/ca_recruit_rushers.lua

@@ -1,11 +1,23 @@
 -- Make the generic_recruit_engine functions work as external CAs
 
+local ca_castle_switch
+for ai_tag in wml.child_range(wesnoth.sides[wesnoth.current.side].__cfg, 'ai') do
+    for stage in wml.child_range(ai_tag, 'stage') do
+        for ca in wml.child_range(stage, 'candidate_action') do
+            if ca.location and string.find(ca.location, 'ca_castle_switch') then
+                ca_castle_switch = wesnoth.require("ai/lua/ca_castle_switch.lua")
+                break
+            end
+        end
+    end
+end
+
 local dummy_engine = { data = {} }
-local ca_castle_switch = wesnoth.require("ai/lua/ca_castle_switch.lua")
-local params = {
-    score_function = (function() return 300000 end),
-    min_turn_1_recruit = (function() return ca_castle_switch:evaluation({}, dummy_engine.data) > 0 end),
-    leader_takes_village = (function()
+
+local params = { score_function = (function() return 300000 end) }
+if ca_castle_switch then
+    params.min_turn_1_recruit = (function() return ca_castle_switch:evaluation({}, dummy_engine.data) > 0 end)
+    params.leader_takes_village = (function()
             if ca_castle_switch:evaluation({}, dummy_engine.data) > 0 then
                 local take_village = #(wesnoth.get_villages {
                     x = dummy_engine.data.leader_target[1],
@@ -16,7 +28,8 @@ local params = {
             return true
         end
     )
-}
+end
+
 wesnoth.require("ai/lua/generic_recruit_engine.lua").init(dummy_engine, params)
 
 local ca_recruit_rushers = {}