Add missing boost header

Some boost headers were remove from gazebo7 header files
and gazebo_ros_joint_state_publisher.cpp was using it
implicitly.

gazebo_plugins/src/gazebo_ros_joint_state_publisher.cpp

@@ -25,6 +25,7 @@
  *  SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  **/
+#include <boost/algorithm/string.hpp>
 #include <gazebo_plugins/gazebo_ros_joint_state_publisher.h>
 #include <tf/transform_broadcaster.h>
 #include <tf/transform_listener.h>