meson: use proper handling of wayland-protocols dependency

meson: use proper handling of wayland-protocols dependency

Ensure that resolution of the subproject occurs via the dependency
interface, not the "poke at subprojects manually" interface, and make
that actually work via --wrap-mode=forcefallback.

There's no need to mark it as not-required and then manually invoke
subproject(), since fallback should work correctly and it is always
needed.

However, if fallback was performed (or forced) it would error out since
get_variable() was instructed to only use pkg-config while the relevant
variable was exported by the subproject as an internal fallback
dependency.