Enable 'PolyKinds' in Linear.Trace (#122)

This turns on 'PolyKinds' for appropriate GHC versions in 'Linear.Trace'.
The particular motivation is to generalize the `instance Dim * n =>
Trace (V * n)` to the kind `Nat`, such that `trace m` compiles for types
like `m :: V 5 (V 5 n)`.

Fixes #121

src/Linear/Trace.hs

@@ -1,6 +1,9 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE DefaultSignatures #-}
+#if __GLASGOW_HASKELL__ >= 707
+{-# LANGUAGE PolyKinds #-}
+#endif
 #if __GLASGOW_HASKELL__ >= 702 && __GLASGOW_HASKELL__ < 710
 {-# LANGUAGE Trustworthy #-}
 #endif