gemspec: fix io-event version lower than v1.11.0

It appears io-event 1.11.0 ~ 1.12.1 are unstable on Windows.
See #5041.

Signed-off-by: Daijiro Fukuda <fukuda@clear-code.com>

Author: daipom

fluentd.gemspec

@@ -42,6 +42,10 @@ Gem::Specification.new do |gem|
   gem.add_runtime_dependency("uri", '~> 1.0')
   gem.add_runtime_dependency("async-http", "~> 0.86")
 
+  # It appears io-event 1.11.0 ~ 1.12.1 are unstable on Windows.
+  # See https://github.com/fluent/fluentd/issues/5041.
+  gem.add_runtime_dependency("io-event", "< 1.11.0")
+
   # gems that aren't default gems as of Ruby 3.4
   gem.add_runtime_dependency("base64", ["~> 0.2"])
   gem.add_runtime_dependency("csv", ["~> 3.2"])