qtbase: add libxext dependency for xcb PACKAGECONFIG
qtbase: add libxext dependency for xcb PACKAGECONFIG

* with new Qt, qtbase fails to build with x11 DISTRO_FEATURES
  ERROR: Feature 'xcb-xlib' was enabled, but the pre-condition 'features.xlib && libs.xcb_xlib' failed.

  Trying source 0 (type makeSpec) of library xlib ...
  None of [libXext.so libXext.a] found in [] and global paths.

Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

sapiippo/meta-qt5,sapiippo/meta-qt5,schnitzeltony/meta-qt5,meta-qt5/meta-qt5,meta-qt5/meta-qt5,schnitzeltony/meta-qt5