seat: fallback to v6 if libwayland 1.17 isn't available
seat: fallback to v6 if libwayland 1.17 isn't available

See https://github.com/swaywm/wlroots/pull/1675#issuecomment-487300445

swaywm/wlroots,ascent12/wlroots,ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots