libftdi: bump to version 0.20

Needed for openocd 0.7.0 (added in the next commit).

NOTE: There is also a libftdi 1.0 version, but openocd only needs 0.20
so I didn't bother. (libftdi 1.0 uses CMake build system and has lots of
changes.)

pkgs/development/libraries/libftdi/default.nix

@@ -1,11 +1,11 @@
 {stdenv, fetchurl, libusb}:
 
 stdenv.mkDerivation rec {
-  name = "libftdi-0.17";
+  name = "libftdi-0.20";
   
   src = fetchurl {
     url = "http://www.intra2net.com/en/developer/libftdi/download/${name}.tar.gz";
-    sha256 = "1w5bzq4h4v9qah9dx0wbz6w7ircr91ack0sh6wqs8s5b4m8jgh6m";
+    sha256 = "13l39f6k6gff30hsgh0wa2z422g9pyl91rh8a8zz6f34k2sxaxii";
   };
 
   buildInputs = [ libusb ];