Fix GHC 9.2 build

Close #1513.

GHC 9.2 needs explicit kind signature here, I don't really understand
why.

This kind signature is correct and not too restritive, because `HasLink`
is technically defined `class HasLink endpoint` which means that it is
infered as `k -> Constraint`. In the instance signature, we have
`HasLink ((arr :: a -> b) :> sub)`, so here the `k` is the same kind as
the one of `:>` which is not polykinded.

Author: guibou

servant/src/Servant/Links.hs

@@ -649,7 +649,7 @@ simpleToLink _ toA _ = toLink toA (Proxy :: Proxy sub)
 -- Erroring instance for 'HasLink' when a combinator is not fully applied
 instance TypeError (PartialApplication HasLink arr) => HasLink ((arr :: a -> b) :> sub)
   where
-    type MkLink (arr :> sub) _ = TypeError (PartialApplication HasLink arr)
+    type MkLink (arr :> sub) _ = TypeError (PartialApplication (HasLink :: * -> Constraint) arr)
     toLink = error "unreachable"
 
 -- Erroring instances for 'HasLink' for unknown API combinators