misc/sdformat: Fix build with Ruby 3.x

Since Ruby 3.0 rexml is converted to bundled gem. On the other hand
our Ruby ports (lang/rubyXY) don't install bundled gems. So rexml
isn't included in packages of Ruby 3.x and ports that require rexml
need to add textproc/rubygem-rexml to their dependency explicitly.

PR: 262506
Approved by: maintainer