Provides ruby2_keywords

So that requiring it succeeds even if that forward compatibility
gem is not installed.

version.c

@@ -86,6 +86,8 @@ Init_version(void)
      * The version of the engine or interpreter this ruby uses.
      */
     rb_define_global_const("RUBY_ENGINE_VERSION", (1 ? version : MKSTR(version)));
+
+    rb_provide("ruby2_keywords.rb");
 }
 
 #if USE_MJIT