add gtk4 package (#1434)

packages/g/gtk+4/xmake.lua

@@ -0,0 +1,14 @@
+package("gtk+4")
+
+    set_homepage("https://gtk.org/")
+    set_description("Toolkit for creating graphical user interfaces")
+    set_license("LGPL-2.0-or-later")
+
+    if on_fetch then
+        on_fetch("linux", function (package, opt)
+            if opt.system and package.find_package then
+                return package:find_package("pkgconfig::gtk4")
+            end
+        end)
+    end
+