Add base64 to the gemspec

[hswong3i]

base64 will no longer be the default gem in the Ruby 3.4, without this patch following warning message may come from ruby 3.3+

/opt/vagrant/lib/vagrant/util/powershell.rb:4: warning: base64 was
loaded from the standard library, but will no longer be part of the
default gems since Ruby 3.4.0. Add base64 to your Gemfile or gemspec.

See https://github.com/thinreports/thinreports-generator/pull/134

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