clementine: fix evaluation

$ nix-env -f . -qa '*' --meta --xml --drv-path --show-trace
error: while querying the derivation named `clementine-1.2.1':
while evaluating `optional' at .../lib/lists.nix:113:20, called from .../pkgs/applications/audio/clementine/default.nix:50:22:
undefined variable `not' at .../pkgs/applications/audio/clementine/default.nix:50:32