Explicitly import GenericMode typeclass method

Author: dalpd

servant-server/example/greet.hs

@@ -17,6 +17,7 @@ import           Network.Wai
 import           Network.Wai.Handler.Warp
 
 import           Servant
+import           Servant.API.Generic ((:-))
 import           Servant.Server.Generic ()
 
 -- * Example