x11-misc/xscreensaver: Depend on x11-libs/gdk-pixbuf[X], fixes #608982

x11-misc/xscreensaver: Depend on x11-libs/gdk-pixbuf[X], fixes #608982

Configure step gives following warning otherwise.

    Warning: GTK version 2.24.31 was found, but at least one supporting
             library (gdk-pixbuf-xlib-2.0) was not, so GTK can't be used.
             Perhaps some of the development packages are not installed?

    Warning: The GTK libraries do not seem to be available; the
             `xscreensaver-demo' program requires them.

    Warning: Use of GDK-Pixbuf was requested, but it was not found.
             The XPM library is being used instead.

             Some of the demos will not use images as much as they could.
             You should consider installing GDK-Pixbuf and re-running
             configure.  (GDK-Pixbuf is recommended over XPM, as it
             provides support for more image formats.)

Gentoo-Bug: https://bugs.gentoo.org/608982
Package-Manager: Portage-2.3.3, Repoman-2.3.1
Closes: https://github.com/gentoo/gentoo/pull/4091