gtkprintbackendcups.c: fix \n at end of a debugging note
Without the newline in the debugging message, there is a mess because two messages get into the same line, as it was for me in https://bugzilla.gnome.org/show_bug.cgi?id=793529 .
Without the newline in the debugging message, there is a mess because two messages get into the same line, as it was for me in https://bugzilla.gnome.org/show_bug.cgi?id=793529 .