slave: Simple fix for changed type signature of #tar_out method.

slave: Simple fix for changed type signature of #tar_out method.

Because optional arguments have been added, the type signature
changed:

File "slave.ml", line 323, characters 17-26:
Error: This expression has type string -> string -> unit
       but an expression was expected of type
         ?compress:string ->
         ?numericowner:bool ->
         ?excludes:string array -> string -> string -> unit