bsd.lib.mk: Do not include bsd.incs.mk for INTERNALLIB

If we're building an internal lib do not bother including bsd.incs.mk so we
will not install the headers.
This also "solves" a problem with pkgbase where a libXXX-development package
is created and due to how packages are created we add a dependency to a
libXXX package that doesn't exists.

Reported by: pizzamig
Reviewed by: pizzamig bapt emaste
Differential Revision: https://reviews.freebsd.org/D24166