xtreemfs: unbreak by pinning boost

pkgs/by-name/xt/xtreemfs/package.nix

···
   stripJavaArchivesHook,
   ant,
   attr,
-  boost,
+  boost186,
   cmake,
   file,
   fuse,
···
   valgrind,
   which,
 }:
+
+let
+  boost = boost186;
+in
 
 stdenv.mkDerivation {
   pname = "XtreemFS";