To use Array instance for Gem::Specification#extensions

Author: hsbt

gdbm.gemspec

@@ -16,7 +16,7 @@ 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/gdbm/extconf.rb"
+  spec.extensions    = ["ext/gdbm/extconf.rb"]
   spec.required_ruby_version = ">= 2.5.0dev"
 
   spec.add_development_dependency "bundler", "~> 1.14"