Remove extraneous rbtrace files from build

This saves about 8 MB of space occupied by the msgpack .tar.gz and
source code.

changelogs/unreleased/sh-remove-rbtrace-files.yml

+---
+title: Remove extraneous rbtrace files from build
+merge_request: 4647
+author:
+type: changed

config/projects/gitlab.rb

@@ -214,6 +214,8 @@ exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/core'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/ext'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/src/ruby/spec'
 exclude 'embedded/lib/ruby/gems/*/gems/grpc-*/third_party'
+exclude 'embedded/lib/ruby/gems/*/gems/rbtrace-*/ext/src'
+exclude 'embedded/lib/ruby/gems/*/gems/rbtrace-*/ext/dst'
 # Enable signing packages
 package :rpm do