Hi, I'm trying the tutotial on http://gazebosim.org/tutorials?tut=guioverlay&cat=userinput on using the gui interfaces but when I enter the space of "make" after building the packages the following error is displayed:

/home/juantelo/gazebo_gui_spawn/build/../GUIExampleSpawnWidget.hh:21:35: fatal error: gazebo/gui/GuiPlugin.hh: No such file or directory
#include <gazebo/gui/GuiPlugin.hh>

I don't have that file anywhere and I want to know where can I find it?