libxkbcommon: Add PACKAGECONFIG for x11

* allow to explicitly disable x11 with --disable-x11, otherwise
  do_configure fails for DISTROs without x11 in DISTRO_FEATURES:
  | No package 'xcb-xkb' found
  | configure:18763: $? = 1
  | configure:18777: result: no
  | No package 'xcb' found
  | No package 'xcb-xkb' found
  | configure:18793: error: xkbcommon-x11 requires xcb-xkb >= 1.10 which
    was not found. You can disable X11 support with --disable-x11.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>