export HiddenWindows type constructor

Some users like to include type signatures in their configuration. The
HiddenWindows type constructor was not exported, making it impossible to
write a type signature when using hiddenWindows.

With this change, a user can write an xmonad.hs like:

  import XMonad
  import XMonad.Layout.Hidden (HiddenWindows, hiddenWindows)
  import XMonad.Layout.LayoutModifier

  myLayout :: ModifiedLayout
              HiddenWindows
              (Choose Tall (Choose (Mirror Tall) Full))
              Window
  myLayout = hiddenWindows $ layoutHook def

  main :: IO ()
  main = xmonad def { layoutHook = myLayout }

XMonad/Layout/Hidden.hs

@@ -20,7 +20,8 @@
 module XMonad.Layout.Hidden
        ( -- * Usage
          -- $usage
-         HiddenMsg (..)
+         HiddenWindows
+       , HiddenMsg (..)
        , hiddenWindows
        , hideWindow
        , popOldestHiddenWindow