Gemfile: Avoid installing rdoc in TruffleRuby.

Avoid installing rdoc in TruffleRuby as a workaround of the truffleruby-head
version 24.0.0-dev issue.

Author: junaruga

Gemfile

@@ -8,5 +8,8 @@ group :development do
   gem "test-unit", "~> 3.0", ">= 3.4.6"
   gem "test-unit-ruby-core"
   # In the case of Ruby whose rdoc is not a default gem.
-  gem "rdoc"
+  # Avoid installing rdoc gem in TruffleRuby as a workaround of the following
+  # issue.
+  # https://github.com/ruby/openssl/issues/699
+  gem "rdoc" unless RUBY_ENGINE == "truffleruby"
 end