package/qt5base: fix xkbcommon option

package/qt5base: fix xkbcommon option

- Qt 5.6: according to configure --help the command line option to
  enable x11 xkbcommon support is '-system-xkbcommon-x11'

- Qt 5.12: according to configure --help the command line option to
  enable xkbcommon support is '-xkbcommon'

Fixes [1]:

  ERROR: Invalid value given for boolean command line option 'xkbcommon'.

[1] http://autobuild.buildroot.net/results/87076d03bb92a9940447ca80fc1f202e35ef39da

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>