backend-headless needs X11/Xlib.h

• Meson config:

    --prefix="${BUILD_PREFIX}"
    -Dlibdir=${BUILD_PREFIX}/lib
    -Dcolor-management-lcms=false
    -Dcolor-management-colord=false
    -Dshell-ivi=false
    -Dtest-junit-xml=false
    -Ddemo-clients=false
    -Dsimple-clients=
    -Dxwayland=false
    -Dbackend-fbdev=false
    -Dbackend-rdp=false
    -Dbackend-x11=false
    -Dbackend-drm-screencast-vaapi=false
    -Dweston-launch=false
    -Dshell-desktop=false
    -Ddesktop-shell-client-default=weston-fullscreen-shell
    -Dimage-webp=false
    -Dlauncher-logind=false -Dsystemd=false -Dremoting=false
    -Dpipewire=false
    -Dsimple-dmabuf-drm=intel
    -Dtools=

• Up to 20b092d8 the build is successful

• The changes in 9adfe7b9 make build unsuccessful:

x86_64-mdi-linux-gnu-gcc -Ilibweston/backend-headless/2487230@@headless-backend@sha -Ilibweston/backend-headless -I../libweston/backend-headless -I. -I../ -Iinclude -I../include -Ilibweston -I../libweston -Iprotocol -I/home/ahe/meetwise/mdi/.build/decoder-image-stage/usr/include/pixman-1 -I/home/ahe/meetwise/mdi/.build/decoder-image-stage/usr/include/libdrm -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Wextra -Wpedantic -std=gnu99 -g -Wno-unused-parameter -Wno-shift-negative-value -Wno-missing-field-initializers -Wno-pedantic -fvisibility=hidden --sysroot=/home/ahe/meetwise/mdi/.build/decoder-image-stage '-DNDEBUG -fno-omit-frame-pointer -fPIC -g -march=skylake -O3 -pipe' -UNDEBUG -fPIC -MD -MQ 'libweston/backend-headless/2487230@@headless-backend@sha/headless.c.o' -MF 'libweston/backend-headless/2487230@@headless-backend@sha/headless.c.o.d' -o 'libweston/backend-headless/2487230@@headless-backend@sha/headless.c.o' -c ../libweston/backend-headless/headless.c
In file included from /home/ahe/meetwise/mdi/.build/decoder-image-stage/usr/include/EGL/egl.h:39,
                 from ../libweston/renderer-gl/gl-renderer.h:36,
                 from ../libweston/backend-headless/headless.c:42:
/home/ahe/meetwise/mdi/.build/decoder-image-stage/usr/include/EGL/eglplatform.h:130:10: fatal error: X11/Xlib.h: No such file or directory
  130 | #include <X11/Xlib.h>

Is this expected behavior?