Added mission extension configuration

Author: hsbt

nkf.gemspec

@@ -31,4 +31,5 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+  spec.extensions    = ["ext/nkf/extconf.rb"]
 end