port: Fix build error when building in a flatpak

If you create a project with gnome builder and port it blueprint and try to build it in flatpak you will get a compiler error that /org/gnome/Example/window.ui.
Setting the source_dir variable in meson fixes this.

blueprintcompiler/interactive_port.py

@@ -266,6 +266,7 @@ blueprints = custom_target('blueprints',
 arguments in {Colors.UNDERLINE}{meson_file}{Colors.NO_UNDERLINE}:{Colors.CLEAR}
 
 dependencies: blueprints,
+source_dir: meson.current_build_dir(),
     """
                     )
                     enter()