build: subproject support for wayland-protocols
As in wlroots: [1]
[1]:
-wl_protocol_dir = wayland_protos.get_variable(pkgconfig: 'pkgdatadir')
+wl_protocol_dir = wayland_protos.get_variable('pkgdatadir')
wayland_scanner_dep = dependency('wayland-scanner', required: false, native: true)
if wayland_scanner_dep.found()
