xapp_xfs: Fix compile errors after libXfont 1.5.0 update

Fixes
http://autobuild.buildroot.net/results/385/385cffbab4261337afa449998aa7457a8142db92/



[Thomas: add comment about the new xfont_font-util dependency being
needed only for autoreconf.]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>