Reduce the duplicated 'nss' in include and lib path

From jbeich:
This appeared after r78824 which failed to adjust do-install. Arch, Debian,
Gentoo, OpenBSD use include/nss/, Fedora uses include/nss3/, NetBSD/PkgSrc
uses include/nss/nss/.

PR: 243835
Reviewed by: jbeich
Differential Revision: https://reviews.freebsd.org/D23424