backend-wayland: fix incorrect header

The wayland.c actually include 'xdg-shell-client-protocol.h' instead of
the server one, so fix it. Otherwise, it's possible to get build failure
due to race condition.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Daniel Stone <daniels@collabora.com>
[daniels: Found in OpenEmbedded/Yocto source.]