thread_pthread.c: fix compile error on HP-UX
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42808 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
thread_pthread.c: fix compile error on HP-UX
[ruby-core:56983] [Feature #8793]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42808 b2dd03c8-39d4-4d8f-98ff-823fe69b080e