REGRESSION(r166779): [GTK] Printing doesn't work since r166779
https://bugs.webkit.org/show_bug.cgi?id=131725

Reviewed by Philippe Normand.

* Source/cmake/OptionsGTK.cmake: Check GTK_UNIX_PRINT_FOUND
instead of GTK_UNIX_PRINTING_FOUND and set the macro
HAVE_GTK_UNIX_PRINTING to 1 when found.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@167344 268f45cc-cd09-0410-ab3c-d52691b4dbfc