watchman: add update script

(cherry picked from commit ec166bb5f0)
(cherry picked from commit 5e28ca8e9e03a297142e4dd8cd42c4b857e71642)

pkgs/by-name/wa/watchman/package.nix

@@ -31,6 +31,8 @@
 
   gtest,
 
+  nix-update-script,
+
   stateDir ? "",
 }:
 
@@ -105,6 +107,8 @@ stdenv.mkDerivation (finalAttrs: {
     remove-references-to -t ${folly.fmt.dev} $out/bin/*
   '';
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "Watches files and takes action when they change";
     homepage = "https://facebook.github.io/watchman";