[ruby/yaml] Make YAML::DBM optional

https://github.com/ruby/yaml/commit/f7c44d3fd7

lib/yaml/dbm.rb

@@ -1,6 +1,10 @@
 # frozen_string_literal: false
 require 'yaml'
-require 'dbm'
+
+begin
+  require 'dbm'
+rescue LoadError
+end
 
 module YAML
 
@@ -288,4 +292,4 @@ class DBM < ::DBM
     alias :each :each_pair
 end
 
-end
+end if defined?(DBM)

test/yaml/test_dbm.rb

@@ -43,4 +43,4 @@ class TestYAMLDBM < Test::Unit::TestCase
     @dbm["key"] = "value"
     assert_equal ["key", "value"], @dbm.shift
   end
-end
+end if defined?(YAML::DBM)