Build.xml now supports "include" inside "files"

Hello,

I needed this because I want my native extensions to expose headers (or any other compile dependencies) that can be used in other native extensions, or even in haxe inline cpp code.

Basically now you can do something like:
somelib/native.xml:
```
<xml>
      <compilerflag value="-I${haxelib:somelib}/project/include" />
</xml>
```
anotherlib/File.hx:
```
(...)
@:buildXml('
	<files id="haxe">
		<include name="${haxelib:somelib}/native.xml" />
	</files>
')
(...)
```

evenanotherlib/project/Build.xml:
```
(...)
<files id="src">
        (...)
	<include name="${haxelib:somelib}/native.xml" />
        (...)
</files>
(...)
```

Maybe there is a better way of doing this... But still this works, and I think its a quite harmless addition to the BuildTool.

Thanks!