configure.in: check for sys/pstat.h

* configure.in (sys/pstat.h): fix missing header check for
  missing/setproctitle.c on HP-UX.  [ruby-core:56644] [Bug #8792]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42713 b2dd03c8-39d4-4d8f-98ff-823fe69b080e