meson.build: update wayland requirement to 1.18
meson.build: update wayland requirement to 1.18

This updates the version requirement for wayland-server to 1.18, which
is needed for wl_global_remove and wl_global_set_user_data

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