package/Config.in: hide X.org subsections if XORG7 isn't enabled

package/Config.in: hide X.org subsections if XORG7 isn't enabled

(most of) the individual packages depends on BR2_PACKAGE_XORG7, but the
subsection comments do not, so we end up displaying them when Xorg isn't
enabled which isn't very useful:

[ ] X.org X Window System  ----
    *** X applications ***
    *** X libraries and helper libraries ***
    *** X window managers ***

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>