x11-themes/gtk-arc-themes: Try to fix build order bug (D29353)

Uncovered while building with devel/samurai but can probably also
happen with ninja eventually.

samu: job failed: /usr/local/bin/glib-compile-resources --sourcedir=common/gtk-3.0 --target=common/gtk-3.0/gtk-lighter.gresource common/gtk-3.0/gtk-lighter.gresource.xml
common/gtk-3.0/gtk-lighter.gresource.xml: Failed to locate checkbox-checked.png

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-05_21h03m50s/logs/errors/gtk-arc-themes-20210127.log

PR:		254678