qtcreator: add qtdeclarative to QML2_IMPORT_PATH

When building with Qt 5.4 this is not a problem, but with Qt 5.5 (tested
locally), it is clear that this dependency is missing:

$ ./result/bin/qtcreator
file:///nix/store/zmpf6ydrjdydd85wh6splpywv6aj4782-qtcreator-3.4.2/share/qtcreator/welcomescreen/welcomescreen.qml:31:1:
module "QtQuick" is not installed
     import QtQuick 2.1
     ^

With Qt 5.4 something else is pulling in the qtdeclarative dependency,
it can be seen in the qtcreator wrapper script.

Note to self: "import QtQuick" != qtquickcontrols. QtQuick is
apparently located in the qtdeclarative module.