gemspec: add logger gem as runtime dependency

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

fluent-logger.gemspec

@@ -22,6 +22,10 @@ Gem::Specification.new do |gem|
   gem.license       = "Apache-2.0"
 
   gem.add_dependency "msgpack", ">= 1.0.0", "< 2"
+
+  # logger gem that isn't default gems as of Ruby 3.5
+  gem.add_runtime_dependency("logger", ["~> 1.6"])
+
   gem.add_development_dependency 'rake', '>= 0.9.2'
   gem.add_development_dependency 'rspec', '>= 3.0.0'
   gem.add_development_dependency 'rspec-its', '>= 1.1.0'