bsd.nls.mk isn't optional.

It is protected by MK_NLS. If it should really be optional then
it needs to be documented as such in share/mk/bsd.README and
.sinclude used where needed.

This fixes a regression from r335011.

PR: 232527
Submitted by: jarrod@downtools.com.au
Reported by: ktullavik@gmail.com
MFC after: 3 days