Fix a typo in commit fa1ca8070

In commit "awful.client.setslave: Fix for not-visible clients", I copied some
code from awful.client.visible(). However, I missed that I where supposed to
change something in the code that I had copied. Whoops.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/client.lua.in

@@ -412,7 +412,7 @@ end
 --- Set the client as slave: put it at the end of other windows.
 -- @param c The window to set as slave.
 function setslave(c)
-    local cls = capi.client.get(screen)
+    local cls = capi.client.get(c.screen)
     for k, v in pairs(cls) do
         c:swap(v)
     end