python3Packages.soundfile: fix cross-compilation

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

···
 
     checkInputs = [ pytest ];
     propagatedBuildInputs = [ numpy libsndfile cffi ];
+    propagatedNativeBuildInputs = [ cffi ];
 
     meta = {
       description = "An audio library based on libsndfile, CFFI and NumPy";