Fix missing Semigroup in ghc 8.0

Prelude in ghc 8.0 does not come with Semigroup. Importing it directly
works in 8.0, 8.2, and 8.4.

src/Reflex/Time.hs

@@ -28,6 +28,7 @@ import Control.Monad.IO.Class
 import Data.Align
 import Data.Data (Data)
 import Data.Fixed
+import Data.Semigroup
 import Data.Sequence (Seq, (|>))
 import qualified Data.Sequence as Seq
 import Data.These
@@ -369,4 +370,3 @@ tickInfo_alreadyElapsed :: Lens' TickInfo NominalDiffTime
 tickInfo_alreadyElapsed f (TickInfo x1 x2 x3) = (\y -> TickInfo x1 x2 y) <$> f x3
 {-# INLINE tickInfo_alreadyElapsed #-}
 #endif
-