Bump haddock-api version to 2.17.3.1

This is a special out-of-band patch release addressing haskell#571

Author: hvr

haddock-api/haddock-api.cabal

@@ -1,5 +1,5 @@
 name:                 haddock-api
-version:              2.17.3
+version:              2.17.3.1
 synopsis:             A documentation-generation tool for Haskell libraries
 description:          Haddock is a documentation-generation tool for Haskell
                       libraries