Fix ruby 3.4 warning

[kzkn]

• fiddle gem warning
• frozen string warning

[win93]

For reference, this is the warnings you get without this patch:

.../ruby-xz/lib/xz.rb:314: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
.../lib/xz.rb:206: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
.../ruby-xz/lib/xz/stream.rb:115: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

This fix looks good.

Testing this patch passes on ruby-3.4, but I have latent maintenance work to get GitLab CI working again to test other rubies before I'll merge this PR.

(Apologies for the delay, I found out that GitLab was not emailing me about this project, but I've addressed this problem 😉)

[jer-k]

The fiddle warning is now a breaking change in Ruby 4.0.0 as noted in #13.

Anything we can do to help here?