fix GtkStyleProvider constants namespace

[d47081]

NS update for #121

<?php

\Gtk::init();

var_dump(\GtkStyleProvider::PRIORITY_APPLICATION); // int(600)
var_dump(\GtkStyleProvider::PRIORITY_FALLBACK); // int(1)
var_dump(\GtkStyleProvider::PRIORITY_SETTINGS); // int(400)
var_dump(\GtkStyleProvider::PRIORITY_THEME); // int(200)
var_dump(\GtkStyleProvider::PRIORITY_USER); // int(800)

\Gtk::main();

p.s. forgot to disable empty lines filter in vscode - but there is only one line :)