remove unnecessary include to silence warning in the compilation process

Any compilation unit having included QT OpenGL stuff should not get in
touch with anything that has classic OpenGL headers included...

modules/gui/src/tools/measure_tool.cc

@@ -31,8 +31,6 @@
 #include <ost/platform.hh>
 #include <ost/gfx/scene.hh>
 
-#include <ost/gui/gl_canvas.hh>
-
 #include <ost/geom/vec3.hh>
 
 #include <QDir>