poppler: fix select of Qt4 GUI module
For some silly reason, the option is named BR2_PACKAGE_QT_GUI_MODULE
and not BR2_PACKAGE_QT_QUI.
Hopefully fixes:
http://autobuild.buildroot.org/results/47d/47dee003b29f156496492757cbaa4208b002ab65/
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>