qt54: add missing mesa include dir

Try to build e.g. the Qt5 Camera Example[1] and see that qmake fails to
find <GL/gl.h>. This fixes it.

[1] http://doc.qt.io/qt-5/qtmultimediawidgets-camera-example.html
(Although since nixpkgs qtcreator still lacks 'examples', we have to
download the sources manually and use "qmake && make".)