gnuplot: gd support needs libpng

GD support in gnuplot needs PNG support in GD.
So make sure we are getting the full package to enable it, fixes:
http://autobuild.buildroot.net/results/678bd3babfcb3535e3a9ed1e111f741c3d3101f3/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>