Add extension parameter to parent tmpfile method signature

Even though this doesn't implement it, it's still good to use the same
signature consistently.

Fixes: 37dd194 ("Support an extension to tmpfile")

lib/beaker/host.rb

@@ -544,7 +544,7 @@ def do_rsync_to from_path, to_path, opts = {}
       raise Beaker::Host::CommandFailure, result.error
     end
 
-    def tmpfile(name = '')
+    def tmpfile(name = '', extension = nil)
       raise NotImplementedError
     end