Add dependency on gazebo plugins

[rkoyama1623]

Hi, I am using this repository in a Docker environment osrf/ros:noetic-desktop and found that the following roslaunch command does not work correctly .

roslaunch turtlebot3_gazebo turtlebot3_world.launch

For example, gazebo did not Publish the /odom topic or subscribe to the /cmd_vel topic.
The reason for this was that the gazebo_plugins package was not installed via rosdep.
This pull request adds a dependency description to package.xml so that gazebo_plugins is installed with rosdep.
Thank you.

[rkoyama1623]

This PR may fix #154.

[GyuH13]

Hello, apologies for the late reply.
The Noetic version is no longer supported for updates, so we will go ahead and close this PR.
We kindly ask you to use the Humble or Jazzy versions.
Thank you for your interest and contribution!