Unguarded dependency on QGLWidget

Compiling Dashel, Enki, and Aseba without Qt, targets/playground/asebasim/robots/e-puck fails trying to include QGLWidget through Enki Viewer:

[ 71%] Building CXX object targets/playground/CMakeFiles/asebasim.dir/robots/e-puck/EPuck.cpp.obj
In file included from /work/aseba/targets/playground/robots/e-puck/../../PlaygroundViewer.h:33:0,
                 from /work/aseba/targets/playground/robots/e-puck/EPuck.cpp:23:
/work/enki/viewer/Viewer.h:38:21: fatal error: QGLWidget: No such file or directory
compilation terminated.

Without Qt, no compilation of a component with a Qt dependency should be attempted.