Enforce internal linkage for classes in test_gripper_controllers.hpp

To avoid problems with HW_IF_POSITION (-Wsubobject-linkage)

Author: mathias-luedtke

gripper_controllers/test/test_gripper_controllers.hpp

@@ -30,6 +30,9 @@ using hardware_interface::HW_IF_POSITION;
 using hardware_interface::HW_IF_VELOCITY;
 using hardware_interface::StateInterface;
 
+namespace
+{
+
 // subclassing and friending so we can access member variables
 class FriendGripperController
 : public gripper_action_controller::GripperActionController<HW_IF_POSITION>
@@ -62,4 +65,6 @@ class GripperControllerTest : public ::testing::Test
   CommandInterface joint_1_pos_cmd_{joint_name_, HW_IF_POSITION, &joint_commands_[0]};
 };
 
+}  // anonymous namespace
+
 #endif  // TEST_GRIPPER_CONTROLLERS_HPP_