4.19.0: exo-open fails to build without gdk-x11 (unprotected `gdk/gdkx.h` include)

When GTK+ is built without X11 support (as well as the rest of Xfce):

make[2]: Entering directory '/tmp/portage/xfce-base/exo-4.19.0/work/exo-4.19.0/exo-open'
x86_64-pc-linux-gnu-gcc -DHAVE_CONFIG_H -I. -I..  -I.. -DG_LOG_DOMAIN=\"exo-open\" -DPACKAGE_LOCALE_DIR=\"/usr/share/locale\" -DNDEBUG -DHAVE_GNUC_VISIBILITY -I/usr/include/gtk-3.0 -I/usr/include/pango-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/atk-1.0 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/harfbuzz -I/usr/include/freetype2 -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/webp -pthread -I/usr/lib64/libffi/include -I/usr/include/fribidi -I/usr/include/libpng16 -I/usr/include/pixman-1 -I/usr/include/gio-unix-2.0 -I/usr/include/xfce4 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/lib64/libffi/include -pthread -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/gio-unix-2.0 -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include -I/usr/lib64/libffi/include -pthread -I/usr/include/libmount -I/usr/include/blkid -march=znver3 -O3 -fdevirtualize-at-ltrans -fipa-pta -fno-semantic-interposition -flto=16 -fuse-linker-plugin -fvect-cost-model=cheap -pipe -Wl,-O1 -Wl,--as-needed -Wl,-z,relro  -Wall -Wextra -Wno-missing-field-initializers -Wno-unused-parameter -Wmissing-declarations -Wmissing-noreturn -Wpointer-arith -Wcast-align -Wformat -Wformat-security -Wformat-y2k -Winit-self -Wmissing-include-dirs -Wundef -Wredundant-decls -Wshadow -Wdeclaration-after-statement -Wnested-externs -Wold-style-definition -DGSEAL_ENABLE -c -o exo_open-main.o `test -f 'main.c' || echo './'`main.c
main.c:603:10: fatal error: gdk/gdkx.h: No such file or directory
  603 | #include <gdk/gdkx.h>
      |          ^~~~~~~~~~~~
compilation terminated.

FWICS the include in exo-open/main.c is not protected by #if defined(GDK_WINDOWING_X11) like the other two instances in exo are.

Original bug report: https://bugs.gentoo.org/924222