Fix GHC < 8.6 build

It turns out we were relying on instances from a transitive dependency
on base-orphans. Added that explicitly so that we don't regress on the
instances provided with old GHCs.

patch.cabal

@@ -39,6 +39,9 @@ library
                , transformers >= 0.5.6.0 && < 0.6
                , witherable >= 0.3 && < 0.4
 
+  if impl(ghc < 8.6) -- really, if base < 8.12
+    build-depends: base-orphans >= 0.8 && < 0.9
+
   exposed-modules: Data.Functor.Misc
                  , Data.Monoid.DecidablyEmpty
                  , Data.Patch

src/Data/Semigroup/Additive.hs

@@ -14,6 +14,10 @@ module Data.Semigroup.Additive
 import Data.Functor.Const (Const (..))
 import Data.Functor.Identity
 import Data.Proxy
+#if !MIN_VERSION_base(4,12,0)
+-- for :*: and :.: semigroup instances
+import Data.Orphans ()
+#endif
 #if !MIN_VERSION_base(4,11,0)
 import Data.Semigroup (Semigroup (..))
 #endif