add logger to the gemspec

Since logger will be unbundled in ruby 3.5.

Thanks Earlopain

See #397

ruby-vips.gemspec

@@ -36,6 +36,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 2.0.0"
 
   spec.add_runtime_dependency "ffi", ["~> 1.12"]
+  spec.add_runtime_dependency "logger"
 
   spec.add_development_dependency "rake", ["~> 12.0"]
   spec.add_development_dependency "rspec", ["~> 3.3"]