package/cegui: disable xerces support

package/cegui: disable xerces support

Disable temporarily xerces's support due to used char16_t type
which is not supported in the currently used cegui version (0-8-7)
due to forced c++03 standard.

Fixes:
 - http://autobuild.buildroot.net/results/ea04be78b31b3409801597fc0ebe04627742c0c8
 - http://autobuild.buildroot.net/results/7de3a07c304e3939ec9fd164328d004a5a9bba6d
 - http://autobuild.buildroot.net/results/472b4e8438c9100e06e401296f0417a463e5fcf1

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>