Bump Wayland Meson dependency version

(see also weston#296 (closed)) We're slowly accumulating tickets that want a newer Meson than we currently depend on:

• !105 (merged) wants 0.56.0
• !104 (merged) wants 0.54.0 (was made conditional on installed version)
• !63 (closed) will probably go away with 0.54.2 or newer (not yet verified)

At some point we'll need to figure out a good time to bump to a newer version, and what version to jump to. Perhaps we should bump weston's at the same time, whenever we decide to do this.