accounts-qt: Fix running of {pre,post}Configure

Commit 0055c6aa

 introduced a new preConfigure hook that sets the right
qmake path. Unfortunately the mkDerivation attributes of accounts-qt
override the whole configurePhase, so this hook isn't run at all.

This fixes the build of accounts-qt and it now successfully compiles on
my machine.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>