Add missing column specifiers in gnuplot latency script

[shino]

Before this PR the script to display latencies via gnuplot does not
recognize min and errors.

Example command line:

% ./priv/gp_latencies.sh -k "min,mean" -t x11
% ./priv/gp_latencies.sh -k "errors" \
     -p "set title \"Error count\"; set ylabel \"Errors\"" -t x11

[slfritchie]

I'll merge today, thanks, Shino!