Can't compile custom plugin

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Hi! I'm doing the Gazebo tutorial on making your own plugin. I'm currently on the Control plugin part, but when I try running make (after cmake ..) I get the following error:

~/Developer/Gazebo/velodyne_plugin/build$ make
Scanning dependencies of target velodyne_plugin
[ 50%] Building CXX object CMakeFiles/velodyne_plugin.dir/velodyne_plugin.cc.o
/home/martint/Developer/Gazebo/velodyne_plugin/velodyne_plugin.cc: In member function ‘virtual void gazebo::VelodynePlugin::Load(gazebo::physics::ModelPtr, sdf::ElementPtr)’:
/home/martint/Developer/Gazebo/velodyne_plugin/velodyne_plugin.cc:23:8: error: invalid use of incomplete type ‘class gazebo::physics::Model’
  _model->GetName() << "]\n";
        ^
In file included from /usr/include/gazebo-7/gazebo/common/Plugin.hh:49:0,
                 from /usr/include/gazebo-7/gazebo/common/common.hh:34,
                 from /usr/include/gazebo-7/gazebo/gazebo_core.hh:19,
                 from /usr/include/gazebo-7/gazebo/gazebo.hh:20,
                 from /home/martint/Developer/Gazebo/velodyne_plugin/velodyne_plugin.cc:4:
/usr/include/gazebo-7/gazebo/physics/PhysicsTypes.hh:37:11: note: forward declaration of ‘class gazebo::physics::Model’
     class Model;
           ^

My code and folder structure is exactly as in the tutorial, and I've installed Gazebo using apt install gazebo7. My OS is Ubuntu 16.04. Gazebo runs fine otherwise, and I've been able to follow several tutorials without errors. Thankful for any help :)