component/gtk: Disable cups as print backend.

GTK tried to compile against libcups when it was present in the system.
This failed the compilation if the includes were not present.

This commit sets "file" as the only backend for printing, thus
explicitly removing cups as a dependency.

See merge request nexedi/slapos!1649

component/gtk/buildout.cfg

@@ -175,7 +175,7 @@ url = https://download.gnome.org/core/44/44.10/sources/gtk+-3.24.41.tar.xz
 md5sum = 2321e8158feac093b6ed82aac66dbbfd
 pkg_config_depends = ${at-spi2-atk:location}/lib/pkgconfig:${at-spi2-core:location}/lib/pkgconfig:${atk:location}/lib/pkgconfig:${dbus:location}/lib/pkgconfig:${gdk-pixbuf:location}/lib/pkgconfig:${gdk-pixbuf:pkg_config_depends}:${libXi:pkg_config_depends}:${libXi:location}/lib/pkgconfig:${libXrandr:pkg_config_depends}:${libXrandr:location}/lib/pkgconfig:${libepoxy:location}/lib/pkgconfig:${libxkbcommon:location}/lib/pkgconfig:${libxkbcommon:pkg_config_depends}:${pango:location}/lib/pkgconfig:${pango:pkg_config_depends}
 configure-command =
-  meson builddir --libdir=lib -Dprefix="@@LOCATION@@" -Dintrospection=false -Dwayland_backend=false -Dxinerama=no -Ddemos=false -Dexamples=false -Dtests=false -Dinstalled_tests=false
+  meson builddir --libdir=lib -Dprefix="@@LOCATION@@" -Dintrospection=false -Dwayland_backend=false -Dprint_backends=file -Dxinerama=no -Ddemos=false -Dexamples=false -Dtests=false -Dinstalled_tests=false
 make-binary = ninja -C builddir
 environment =
   PATH=${gdk-pixbuf:location}/bin:${gettext:location}/bin:${glib:location}/bin:${meson:location}/bin:${ninja:location}/bin:${perl:location}/bin:${pkgconfig:location}/bin:${xz-utils:location}/bin:${python3:location}/bin:%(PATH)s