Wiki should mention to install gnuplot-x11 for gstshark-plot

[willamowius]

The wiki says on https://developer.ridgerun.com/wiki/index.php?title=GstShark_-_Getting_Started to install a number of packages for the plotting on Ubuntu.

For me gstshark-plot failed on Ubuntu 18.04 when running "set terminal x11...", because the gnuplot-x11 package wasn't installed.