src.libnames.mk: Apply -L also for privatelibs

For INTERNALLIBS and regular libs we add -L to specify the library path,
which may be needed if a library is in a non-default path but also means
we can link against those libraries when built independently. Do the
same for privatelibs as well.

Reviewed by: bapt
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48891