python3Packages.fpylll: fix standalone usage

Otherwise, we get:

>>> from fpylll import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/nix/store/2dmfqi6fhmmrp3jgaz2fznasifxkr1zi-python3-3.12.10-env/lib/python3.12/site-packages/fpylll/__init__.py", line 3, in <module>
    from .fplll.integer_matrix import IntegerMatrix
  File "src/fpylll/fplll/integer_matrix.pyx", line 1, in init fpylll.fplll.integer_matrix
ModuleNotFoundError: No module named 'cysignals'

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

@@ -48,7 +48,10 @@ buildPythonPackage rec {
     fplll
   ];
 
-  propagatedBuildInputs = [ numpy ];
+  propagatedBuildInputs = [
+    numpy
+    cysignals
+  ];
 
   nativeCheckInputs = [ pytestCheckHook ];