nbd: 3.24 -> 3.25, switch to github

"First official release on github,
rather than sourceforge (as in the past)",
according to the github tag message in

https://github.com/NetworkBlockDevice/nbd/releases/tag/nbd-3.25

Note that the commit at hand also fixes the build after it
broke due to the update to GLib 2.75.3 ("Log limit exceeded") in

https://github.com/NixOS/nixpkgs/commit/8e5ee715d12efa8fe3d1071d83c141a6a03d328a

pkgs/tools/networking/nbd/default.nix

···
 
 stdenv.mkDerivation rec {
   pname = "nbd";
-  version = "3.24";
+  version = "3.25";
 
   src = fetchurl {
-    url = "mirror://sourceforge/nbd/nbd-${version}.tar.xz";
-    sha256 = "sha256-aHcVbSOnsz917uidL1wskcVCr8PNy2Nt6lqIU5pY0Qw=";
+    url = "https://github.com/NetworkBlockDevice/nbd/releases/download/nbd-${version}/nbd-${version}.tar.xz";
+    hash = "sha256-9cj9D8tXsckmWU0OV/NWQy7ghni+8dQNCI8IMPDL3Qo=";
   };
 
   buildInputs = [ glib gnutls ]