WC2: make carryover compatible with > 3 sides

wml.variables.wc2_player_count is set in a prestart event
this function is used in a recruit event
Thus we can rely on it being set

data/campaigns/World_Conquest/lua/campaign/scenario.lua

@@ -3,7 +3,7 @@ local wc2_scenario = {}
 local on_event = wesnoth.require("on_event")
 
 function wc2_scenario.is_human_side(side_num)
-	return side_num == 1 or side_num == 2 or side_num == 3
+	return side_num <= wml.variables.wc2_player_count
 end
 
 function wc2_scenario.scenario_num()