Is librtabmap_core.so.0.8 exist in /opt/ros/hydro/lib folder?
$ ls /opt/ros/hydro/lib/librtabmap_core*
/opt/ros/hydro/lib/librtabmap_core.so
/opt/ros/hydro/lib/librtabmap_core.so.0.8
/opt/ros/hydro/lib/librtabmap_core.so.0.8.2
Try reinstall ros-hydro-rtabmap-ros:
$ sudo apt-get remove ros-hydro-rtabmap-ros ros-hydro-rtabmap
$ sudo apt-get install ros-hydro-rtabmap-ros