[dev-qt/qtgui:5] Add platformheaders target subdir

The dev-qt/qtgui-5.9999 ebuild was failing to build, complaining that
QtPlatformHeaders/qxcbwindowfunctions.h couldn't be found. Upstream
commit 5c28747 added this header file, so we need to add its subdir to
the ebuild as well so it gets installed properly.