rebar.config: use a fork of rebar3_diameter_compiler
This fixes building on Erlang/OTP 29. A pull request [1] has been
submitted upstream. Once merged, we get switch back to upstream.
[1] https://github.com/carlosedp/rebar3_diameter_compiler/pull/28
Change-Id: I548a3fef6f8a23a548fb6376076b9d4a9345d692
