Add Ord (Tree a) instance.

I assume this is an oversight because we do have `Ord1 Tree`.

Some of you may be interested in knowing that I caught this due to
trying out the the changes discusssed in the thread that began in
https://mail.haskell.org/pipermail/libraries/2020-March/030306.html.

Author: Ericson2314

containers/src/Data/Tree.hs

@@ -97,14 +97,15 @@ data Tree a = Node {
     }
 #ifdef __GLASGOW_HASKELL__
   deriving ( Eq
+           , Ord -- ^ @since 0.6.5
            , Read
            , Show
            , Data
            , Generic  -- ^ @since 0.5.8
            , Generic1 -- ^ @since 0.5.8
            )
 #else
-  deriving (Eq, Read, Show)
+  deriving (Eq, Ord, Read, Show)
 #endif
 
 -- | This type synonym exists primarily for historical