print/hplip: Enable install of static CUPS ppd drivers

This is as per the upstream recommendations here:

http://hplipopensource.com/hplip-web/install/manual/distros/freebsd.html

Reported by: Michael Wagner <mikepwagner@mikepwagner.net>
Approved by: adamw (mentor, implicit)
MFH: 2016Q4