Add fiddle to gemspec dependencies

[tompng]

Fiddle will be bundled gem in Ruby 3.5 ruby/ruby@10ecdeb

Fixes this test failure (irb head ubuntu latest)

Failure: test_backtrace_filtering_with_backtrace_filter(TestIRB::BacktraceFilteringTest):
  </from .*\/irbtest-.*.rb:6:in (`|'Object#)bar'/> was expected to be =~
  <"/home/runner/.rubies/ruby-head/lib/ruby/gems/3.4.0+0/gems/reline-0.5.8/lib/reline/terminfo.rb:2: warning: fiddle was loaded from the standard library, but will no longer be part of the default gems since Ruby 3.5.0. Add fiddle to your Gemfile or gemspec.">.

[st0012]

Let's handle this by updating the tests first.
Now that IRB is a Rails dependency (and depends on Reline) I want to be very cautious on adding new dependencies.
From what I can see, fiddle is not directly depended on by many projects and I'm not familiar with it enough to say it's safe for all platforms that Rails supports.

[tompng]

Fiddle is currently a default gem, expected to be always installed...
But in fact, it has trouble installing, sometimes ruby is installed without fiddle (to avoid fiddle compile failure)
I think we need to add it to dependency before 3.5 release, but we have time to find a way to handle it.