Fix build for GHC 8.2

Author: alanz

hie-plugin-api/Haskell/Ide/Engine/Ghc.hs

@@ -29,6 +29,7 @@ import qualified GhcMod.Error                      as GM
 import qualified GhcMod.Gap                        as GM
 import qualified GhcMod.ModuleLoader               as GM
 import qualified GhcMod.Monad                      as GM
+import           Data.Monoid ((<>))
 import qualified GhcMod.Target                     as GM
 import qualified GhcMod.Types                      as GM
 import qualified GhcMod.Utils                      as GM