meson: Install bash completion helper in prefix
meson: Install bash completion helper in prefix

A regression was causing the helpers to be installed in /share which
would lead to permission denied error or PolicyKit to promtp for
permission. See:

  054fa3aa2 meson: Use new define_variable: feature instead of run_command()

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer