meson: prevent sandbox violation if GStreamer and wayland-protocols are subprojects

Suppose you have a project where GStreamer and wayland-protocols are pulled in as dependencies via .wrap files. In that case, Meson's setup step will fail for gst-plugins-bad with the message "Sandbox violation: Tried to grab file viewporter.xml outside current (sub)project." To avoid this exception, one should use Meson's files and join_paths functions. The suggested solution is identical to how GTK 4 processes Wayland files.