add transformers to setup-depends

per Peter Simons:

Setup.hs depends on 'transformers' for versions of base
prior to 4.9.x so that Control.Monad.IO.Class can be included. Just
adding the library to 'setup-depends' fixes the build with older
compilers like GHC 7.10.3.

git-annex.cabal

@@ -301,7 +301,7 @@ source-repository head
 custom-setup
   Setup-Depends: base (>= 4.6), hslogger, split, unix-compat, process,
     filepath, exceptions, bytestring, directory, IfElse, data-default,
-    utf8-string, Cabal
+    utf8-string, transformers, Cabal
 
 Executable git-annex
   Main-Is: git-annex.hs