thread_pthread.h: remove unneeded semaphore.h include

* thread_pthread.h: remove unneeded semaphore.h include
  We currently do not use POSIX semaphores.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46760 b2dd03c8-39d4-4d8f-98ff-823fe69b080e