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>

package/qt5/qt5base/qt5base.mk

@@ -153,7 +153,13 @@ QT5BASE_CONFIGURE_OPTS += $(if $(BR2_PACKAGE_QT5BASE_DIRECTFB),-directfb,-no-dir
 QT5BASE_DEPENDENCIES   += $(if $(BR2_PACKAGE_QT5BASE_DIRECTFB),directfb)
 
 ifeq ($(BR2_PACKAGE_QT5BASE_XCB),y)
-QT5BASE_CONFIGURE_OPTS += -xcb -system-xkbcommon
+QT5BASE_CONFIGURE_OPTS += -xcb
+ifeq ($(BR2_PACKAGE_QT5_VERSION_5_6),y)
+QT5BASE_CONFIGURE_OPTS += -system-xkbcommon-x11
+else
+QT5BASE_CONFIGURE_OPTS += -xkbcommon
+endif
+
 QT5BASE_DEPENDENCIES   += \
 	libxcb \
 	xcb-util-wm \