print/cups-base: rename ICONS option to X11

Why this change ?

• When compiling print/cups-base, if WITHOUT_X11=yes is present in /etc/make.conf, CUPS still tries to update the "Icons cache".
• For this, it tries to build graphics/gtk-update-icon-cache, which in turns requires graphics/cairo, but Cairo builds without X11 support as requested, and graphics/gtk-update-icon-cache fails to build because it needs X11.

PR: 190434
Submitted by: Tobias Kortkamp <t@tobik.me>
Reported by: hjf@hjf.com.ar