rust: fix missing build requirement xz

As of rust-lang/rust#41600 we now need `xz` to build Rust from HEAD.

Author: brandt

Formula/rust.rb

@@ -27,6 +27,8 @@ class Rust < Formula
   head do
     url "https://github.com/rust-lang/rust.git"
 
+    depends_on "xz" => :build
+
     resource "cargo" do
       url "https://github.com/rust-lang/cargo.git"
     end