Build error when xwayland is disabled.

Disabling xwayland (use -Dxwayland=false option in meson) causes build errors. I think this is caused by the following patch.

commit: 388702c181829a99b1540eeea734dc496f62f7f4
frontend: Explicitly destroy Xwayland from frontend code

Ninja build log:

$ ninja -C build/
ninja: Entering directory `build/'
[27/83] Linking target compositor/libexec_weston.so.0.0.0
FAILED: compositor/libexec_weston.so.0.0.0 
cc  -o compositor/libexec_weston.so.0.0.0 compositor/libexec_weston.so.0.0.0.p/meson-generated_.._.._protocol_text-input-unstable-v1-protocol.c.o compositor/libexec_weston.so.0.0.0.p/meson-generated_.._.._protocol_input-method-unstable-v1-protocol.c.o compositor/libexec_weston.so.0.0.0.p/main.c.o compositor/libexec_weston.so.0.0.0.p/text-backend.c.o compositor/libexec_weston.so.0.0.0.p/config-helpers.c.o compositor/libexec_weston.so.0.0.0.p/weston-screenshooter.c.o -Wl,--as-needed -Wl,--no-undefined -shared -fPIC -Wl,--start-group -Wl,-soname,libexec_weston.so.0 '-Wl,-rpath,$ORIGIN/../libweston:/home/etom/work/wayland/env/lib' -Wl,-rpath-link,/home/etom/work/wayland/src/weston/build/libweston -Wl,-rpath-link,/home/etom/work/wayland/env/lib shared/libshared.a libweston/libweston-13.so.0.0.0 /home/etom/work/wayland/env/lib/libwayland-client.so /home/etom/work/wayland/env/lib/libwayland-server.so /usr/lib/x86_64-linux-gnu/libpixman-1.so /usr/lib/x86_64-linux-gnu/libxkbcommon.so /usr/lib/x86_64-linux-gnu/libinput.so /usr/lib/x86_64-linux-gnu/libevdev.so -ldl -Wl,--end-group -pthread
/usr/bin/ld: compositor/libexec_weston.so.0.0.0.p/main.c.o: in function `wet_main':
/home/etom/work/wayland/src/weston/build/../compositor/main.c:4254: undefined reference to `wet_xwayland_destroy'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.