Port pr #2748 to gazebo9 branch

Author: j-rivero

plugins/SimpleTrackedVehiclePlugin.hh

@@ -60,7 +60,7 @@ namespace gazebo {
   /// <collide_without_contact_bitmask> Collision bitmask that will be set to
   ///     the whole vehicle (default is 1u).
 
-  class SimpleTrackedVehiclePlugin :
+  class GZ_PLUGIN_VISIBLE SimpleTrackedVehiclePlugin :
     public TrackedVehiclePlugin
   {
     public: SimpleTrackedVehiclePlugin() = default;

plugins/TrackedVehiclePlugin.hh

@@ -76,7 +76,7 @@ namespace gazebo
   ///             definition are used.
   /// <robot_namespace>  Namespace used as a prefix for gazebo topic names.
   ///                    Default is the name of the model.
-  class TrackedVehiclePlugin : public ModelPlugin
+  class GZ_PLUGIN_VISIBLE TrackedVehiclePlugin : public ModelPlugin
   {
     /// \brief Default Contstuctor
     public: TrackedVehiclePlugin();

plugins/WheelTrackedVehiclePlugin.hh

@@ -46,7 +46,7 @@ namespace gazebo
   ///               appear multiple times, once for each right wheel).
   /// <default_wheel_radius>  The radius used for wheels where radius
   ///                         autodetection fails (default is 0.5 meters).
-  class GAZEBO_VISIBLE WheelTrackedVehiclePlugin :
+  class GZ_PLUGIN_VISIBLE WheelTrackedVehiclePlugin :
     public TrackedVehiclePlugin
   {
     public: WheelTrackedVehiclePlugin() = default;