Add aarch64-unknown-linux-musl target to RustlerPrecompiled config

in order to generate the checksum for the aarch64-unknown-linux-musl
precompiled package when running
'mix rustler_precompiled.download Mjml --all --print'.

lib/mjml.ex

@@ -7,12 +7,25 @@ defmodule Mjml do
   version = mix_config[:version]
   github_url = mix_config[:package][:links]["GitHub"]
 
+  targets = ~w(
+    arm-unknown-linux-gnueabihf
+    aarch64-apple-darwin
+    aarch64-unknown-linux-gnu
+    aarch64-unknown-linux-musl
+    x86_64-apple-darwin
+    x86_64-pc-windows-gnu
+    x86_64-pc-windows-msvc
+    x86_64-unknown-linux-gnu
+    x86_64-unknown-linux-musl
+  )
+
   use RustlerPrecompiled,
     otp_app: :mjml,
     crate: "mjml_nif",
     base_url: "#{github_url}/releases/download/v#{version}",
     force_build: System.get_env("MJML_BUILD") in ["1", "true"],
-    version: version
+    version: version,
+    targets: targets
 
   @doc """
   Converts an MJML string to HTML content.