wget: --HEAD depends_on 'xz'

- Otherwise bootstrap complains about the 'xz' binary not found.

Closes Homebrew/homebrew#16670.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/wget.rb

@@ -20,6 +20,7 @@ class Wget < Formula
   if build.head?
     depends_on "autoconf" => :build
     depends_on "automake" => :build
+    depends_on "xz" => :build
     depends_on "gettext"
   end