python3Packages.fpylll: fix standalone usage (#418448)

pkgs/development/python-modules/fpylll/default.nix

···
     fplll
   ];
 
-  propagatedBuildInputs = [ numpy ];
+  propagatedBuildInputs = [
+    numpy
+    cysignals
+  ];
 
   nativeCheckInputs = [ pytestCheckHook ];