Turns out weston doesn't install xwayland-api header if Xwayland is not
installed, which is a shame, and should be rectified. Meanwhile, let's
guard access to the header and the API that the header exports.
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I77638b263483628f8cc7a3dcbd08c8947db1d86b