build: depend on generated protocol headers

Fixes the build failure where the protocol headers were not generated at
all before compiling src/.libs/libwayland_server_la-wayland-server.o.

The failure was reproducable by starting fresh with 'git clean -dxf' and
not having any wayland headers installed system-wide.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>