qwt: add version 6.x (we already have 5.x)

'qgis', one of the few 'qwt' dependees in nixpkgs, fails to build with
qwt 6. So I'm not moving the default version away from 5.x. Also, not
changing the default allows easy/safe cherry-picking to the stable
branch.

(cherry picked from commit 70b39119c34fe73f4c0e46dfc8eeed8570ec847d)