Add same comment to thwait.gemspec like other gemspec at r64145

[ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/thwait/thwait.gemspec

@@ -1,6 +1,7 @@
 begin
   require_relative "lib/thwait/version"
 rescue LoadError
+  # for Ruby core repository
   require_relative "version"
 end