gnuplot: added upstream license file and updated recipe to match
gnuplot: added upstream license file and updated recipe to match

This fixes QA warnings about missing license file and updates the license
type to match custom license file.

Upstream-Status: Inappropriate (distribution/packaging fix)

Signed-off-by: Stephen Arnold <fe6214edba4d9b2d20a723cfd9bdf269f8b2c023@gmail.com>
Signed-off-by: Martin Jansa <516df3ff67e1fa5d153800b15fb204c0a1a389dc@gmail.com>

WindRiver-OpenSourceLabs/meta-oe-subset,WindRiver-OpenSourceLabs/meta-oe-subset