chore: upgrade grpc-erl to 0.6.4

apps/emqx_gateway/rebar.config

@@ -1,6 +1,6 @@
 {erl_opts, [debug_info]}.
 {deps, [
-  {grpc, {git, "https://github.com/emqx/grpc-erl", {tag, "0.6.2"}}}
+  {grpc, {git, "https://github.com/emqx/grpc-erl", {tag, "0.6.4"}}}
 ]}.
 
 {plugins, [