gnuplot: update hashes

The gnuplot 4.6.6 tarball was updated because of this:
(based on a diff from the original release and the updated one)
http://thread.gmane.org/gmane.comp.graphics.gnuplot.user/7856

Update hashes accordingly, but really this is all kinds of bad.
Fixes:
http://autobuild.buildroot.net/results/925/925fc731e404282873f2c586c98b05da5ce96ae7/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>