[cpp] keep IntMap.h if we keep the constructor (closes #3373)

std/cpp/_std/haxe/ds/IntMap.hx

@@ -40,7 +40,7 @@ package haxe.ds;
 ")
 @:coreApi class IntMap<T> implements haxe.Constraints.IMap<Int,T> {
 
-	private var h : Dynamic;
+	@:ifFeature("haxe.ds.IntMap.new") private var h : Dynamic;
 
 	public function new() : Void { }