haskell-ghc-mod: patch ghc-process.el to avoid hanging

@peti I've determined that the hang which this fixes does not happen all
of the time (in fact, I've been using ghc-mod for weeks without seeing
it once).  But today it happened again, and it made me realize I had
undone this patch locally.  I'm committing it now because it fixes the
problem completely for me, and certainly cannot hurt.

pkgs/development/libraries/haskell/ghc-mod/default.nix

@@ -21,6 +21,7 @@ cabal.mkDerivation (self: {
   configureFlags = "--datasubdir=${self.pname}-${self.version}";
   postInstall = ''
     cd $out/share/$pname-$version
+    sed -i -e 's/"-b" "\\n" "-l"/"-l" "-b" "\\"\\\\n\\""/' ghc-process.el
     make
     rm Makefile
     cd ..