package/sane-backends: poppler-glib needs cairo

package/sane-backends: poppler-glib needs cairo

- poppler-glib depends on cairo (see poppler-0.84.0/CMakeLists.txt):

  185 else()
  186   set(CAIRO_FEATURE "#undef POPPLER_HAS_CAIRO")
  187   set(ENABLE_GLIB OFF)
  188 endif()

Fixes:

  - http://autobuild.buildroot.net/results/9a345f82c7a010f6b2a3361f25b14caefa9dcbef

  checking for POPPLER_GLIB... no
  configure: error: poppler-glib requested but not found

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>