AIX doesn't set OptionLength for boolean options

	https://rubyci.org/logs/rubyci.s3.amazonaws.com/aix71_ppc/ruby-trunk/log/20180904T103302Z.fail.html.gz
	https://www.ibm.com/support/knowledgecenter/ja/ssw_aix_72/com.ibm.aix.commtrf2/getsockopt.htm

	fix typo

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67228 b2dd03c8-39d4-4d8f-98ff-823fe69b080e