gazebo_opticalFlow_plugin: fix compilation warnings against Gazebo 7
gazebo_opticalFlow_plugin: fix compilation warnings against Gazebo 7

Signed-off-by: Nicolae Rosia <207aa917b021b404c269e8647897b2cc6117052a@gmail.com>

darknight-007/sitl_gazebo,darknight-007/sitl_gazebo,darknight-007/sitl_gazebo,darknight-007/sitl_gazebo