Build devel/libpru only on FreeBSD 10+.

Differential Revision:	https://reviews.freebsd.org/D1298
Reviewed by:	swills
Approved by:	swills


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@374569 35697150-7ecd-e111-bb59-0022644237b5
Assume maintainership.


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@374570 35697150-7ecd-e111-bb59-0022644237b5