hplip: specify cupsbackenddir and cupsfilterdir

Fix QA issue when enable multilib:
ERROR: QA Issue: hplip: Files/directories were installed but not shipped in any package:
  /usr/lib
  /usr/lib/cups
  /usr/lib/cups/filter
  /usr/lib/cups/backend
  /usr/lib/cups/filter/hpcac
  /usr/lib/cups/filter/foomatic-rip-hplip
  /usr/lib/cups/filter/pstotiff
  /usr/lib/cups/filter/hpcups
  /usr/lib/cups/filter/hplipjs
  /usr/lib/cups/filter/hpps
  /usr/lib/cups/filter/.debug
  /usr/lib/cups/filter/.debug/hplipjs
  /usr/lib/cups/filter/.debug/hpcups
  /usr/lib/cups/filter/.debug/hpps
  /usr/lib/cups/backend/hp
  /usr/lib/cups/backend/.debug
  /usr/lib/cups/backend/.debug/hp
Please set FILES such that these items are packaged. Alternatively if they are unneeded, avoid installing them or delete them within do_install.
hplip: 17 installed and not shipped files. [installed-vs-shipped]

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>