gnu: Qt: Update to 5.12.7 [fixes CVE-2020-0569, CVE-2020-0570].
* gnu/packages/qt.scm (qtbase, qtsvg, qtimageformats, qtx11extras,
qtxmlpatterns, qtdeclarative, qtconnectivity, qtwebsockets, qtsensors,
qtmultimedia, qtwayland, qtserialport, qtserialbus, qtwebchannel,
qtwebglplugin, qtwebview, qtlocation, qttools, qtscript, qtquickcontrols,
qtquickcontrols2, qtgraphicaleffects, qtgamepad, qtscxml, qtpurchasing,
qtcanvas3d, qtcharts, qtdatavis3d, qtnetworkauth, qtremoteobjects, qtspeech,
qtwebengine): Update to 5.12.7.