fix: add priv to package

[fahchen]

When I upgrade gRPC from 0.7.0 to 0.8.0, the gRPC compile fails.
I found that the PR #351 introduces priv/templates/service.ex.eex, but it isn’t added to package.

To Reproduce

1. cd examples/helloworld

2. Change grpc source to hex

diff --git a/examples/helloworld/mix.exs b/examples/helloworld/mix.exs
index 497b7b1..7ca52df 100644
--- a/examples/helloworld/mix.exs
+++ b/examples/helloworld/mix.exs
@@ -18,7 +18,7 @@ defmodule Helloworld.Mixfile do
 
   defp deps do
     [
-      {:grpc, path: "../../"},
+      {:grpc, "~> 0.8"},
       {:jason, "~> 1.3.0"},
       {:protobuf, "~> 0.11"},
       {:google_protos, "~> 0.3.0"},

3. mix deps.get && mix compile

==> grpc
Compiling 1 file (.erl)
Compiling 49 files (.ex)

== Compilation error in file lib/grpc/protoc/generator/service.ex ==
** (File.Error) could not read file "grpc/examples/helloworld/_build/dev/lib/grpc/priv/templates/service.ex.eex": no such file or directory
    (elixir 1.15.7) lib/file.ex:358: File.read!/1
    (eex 1.15.7) lib/eex.ex:259: EEx.compile_file/2
    lib/grpc/protoc/generator/service.ex:9: (module)
could not compile dependency :grpc, "mix compile" failed. Errors may have been logged above. You can recompile this dependency with "mix deps.compile grpc --force", update it with "mix deps.update grpc" or clean it with "mix deps.clean grpc"

[polvalente]

Thanks! I'll release 0.8.1 shortly.