Changes

rebar.config: fix invalid branch for rebar3_diameter_compiler.git

The upstream has ranamed 'master' branch to 'main', so the old name
is no longer valid.  This resulted in osmo_dia2gsup build failures.

Change-Id: I21490a837ee039e02dfde3e19ded338cf428d291
Related: https://github.com/carlosedp/rebar3_diameter_compiler/commit/077e7b23d9ad2cf73b1b99abe46a273a520288bd