release-cross: Build `gnu.smbfs.hostDrv', not `gnu.smbfs'.

svn path=/nixpkgs/trunk/; revision=32628

pkgs/top-level/release-cross.nix

···
     gnu.hurdCross = nativePlatforms;
     gnu.mach.hostDrv = nativePlatforms;
     gnu.mig = nativePlatforms;
-    gnu.smbfs = nativePlatforms;
+    gnu.smbfs.hostDrv = nativePlatforms;
 
     coreutils.hostDrv = nativePlatforms;
     ed.hostDrv = nativePlatforms;