Use Rex.sleep instead of select

Author: kenkeiras

modules/auxiliary/scanner/ssh/ssh_enumusers.rb

@@ -136,7 +136,7 @@ def attempt_user(user, ip)
     while attempt_num <= retry_num and (ret.nil? or ret == :connection_error)
 
       if attempt_num > 0
-        select(nil, nil, nil, 2**attempt_num)
+        Rex.sleep(2 ** attempt_num)
         print_debug "Retrying '#{user}' on '#{ip}' due to connection error"
       end