devel/rubygem-oj: Fix build

Currently the header files are not included although there is a
post-install step. However there is a missing line where we need to
create the directories.

This fixes the builds of gitlab-ce.

PR: 269020
Reported by: lysfjord.daniel@smokepit.net
Approved by: portmgr (just-fix-it)
Sponsored by: Bounce Experts