borgbackup: Add llfuse to propagated build inputs if not on darwin

The llfuse package depends on fuse which refuses to build on darwin. But
according to a comment in the setup.py of borgbackup [1] it's ok to leave it out
if it's not available. Most of borgbackup should work without it. Would be great
to make it work on darwin but i am not sure if it's possible to get fuse to work
on darwin. I do not know enough about it ;)

After this modification at least the "borg mount" subcommand is broken due to
the missing llfuse module. But the rest seems to work normally.

[1] 72232a9bd5/setup.py (L32)

pkgs/tools/backup/borg/default.nix

@@ -19,8 +19,8 @@ python3Packages.buildPythonApplication rec {
     lz4 openssl python3Packages.setuptools_scm
   ] ++ stdenv.lib.optionals stdenv.isLinux [ acl ];
   propagatedBuildInputs = with python3Packages; [
-    cython llfuse msgpack
-  ];
+    cython msgpack
+  ] ++ stdenv.lib.optionals (!stdenv.isDarwin) [ llfuse ];
 
   preConfigure = ''
     export BORG_OPENSSL_PREFIX="${openssl.dev}"