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>