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