Bump __FreeBSD_version__ for r343891

This will allow upstream consumers, e.g., capsicum-test and third-party
packages (via ports(7)), to test for a specific __FreeBSD_version__ and
expect renameat(2) to be functional.

PR: 222258
Approved by: emaste (mentor)
Reviewed by: emaste
MFC with: r343891
Differential Revision: https://reviews.freebsd.org/D19154