glusterfs: Fix wrong xattr package: xattr -> pyxattr.

Fixes error

  File "/nix/store/lxpsl84km87xpk59nai6a33ihgpfs7qr-glusterfs-3.10.2/libexec/glusterfs/glusterfind/changelog.py", line 105, in populate_pgfid_and_inodegfid
      file_xattrs = xattr.list(p)
  AttributeError: 'module' object has no attribute 'list'

when using `glusterfind pre`.

pkgs/tools/filesystems/glusterfs/default.nix

@@ -28,7 +28,7 @@ let
       pkgs.flask
       pkgs.prettytable
       pkgs.requests
-      pkgs.xattr
+      pkgs.pyxattr
     ]))
     # NOTE: `python2` has to be *AFTER* the above `python2.withPackages`,
     #       to ensure that the packages are available but the `toPythonPath`