gnome-desktop: add xkeyboard-config dependency

gnome-desktop: add xkeyboard-config dependency

gnome-desktop will fail during do_configure if xkeyboard-config had not already
been built. Please see the following for the error:

| configure: error: Package requirements (gdk-pixbuf-2.0 >= 2.21.3
|                                  gtk+-3.0 >= 3.3.6
|                                  glib-2.0 >= 2.38.0
|                                  gio-2.0 >= 2.38.0
|                                  gsettings-desktop-schemas >= 3.5.91
|                                  xrandr >= 1.3
|                                  xext >= 1.1
|                                  xkeyboard-config
|                                  iso-codes) were not met:
|
| No package 'xkeyboard-config' found

Add dependency of xkeyboard-config to this to resolve this issue.

Signed-off-by: Pushpal Sidhu <psidhu@gateworks.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>