rebar.config: use more recent rebar3_diameter_compiler
This fixes building on Erlang/OTP 29. Our pull request [1] has
been merged upstream, but no release has been tagged yet.
[1] https://github.com/carlosedp/rebar3_diameter_compiler/pull/28
Change-Id: I548a3fef6f8a23a548fb6376076b9d4a9345d692
Related: OS#7025
