qtwayland: respect OE_QMAKE_PATH_QT_HEADERS and use QT_VERSION variable

* QT_VERSION allows to use 5.3.1 e.g. when overriding SRCREV
* ${includedir}/${QT_DIR_NAME} is the default value of OE_QMAKE_PATH_QT_HEADERS, but
  when it's changed by user we should respect it here

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