bring back custom-setup stanza

Now that windows is using unix-compat 0.5, don't need a dep on Win32
or unix in custom-setup, so the stanza can be parsed by cabal's limited
parser again.

Note that unix-compat 0.5 is needed on windows, but the cabal file
hasn't bumped it yet, since that would necessarily affect non-windows
builds. Instead, the stack-windows.yaml makes sure the right version is
pulled in.

This commit was sponsored by Jeff Goeke-Smith on Patreon.

git-annex.cabal

@@ -303,6 +303,10 @@ source-repository head
   type: git
   location: git://git-annex.branchable.com/
 
+custom-setup
+  Setup-Depends: base (>= 4.5), hslogger, split, unix-compat, process,
+    filepath, exceptions, bytestring, directory, IfElse, data-default, Cabal
+
 Executable git-annex
   Main-Is: git-annex.hs
   Build-Depends: