bsd.prog.mk: add "/usr/lib" to list of base system directories

kgzldr.o is installed into /usr/lib but using bsd.prog.mk. Add
/usr/lib to the base system directory list so that debug files are
installed into /usr/lib/debug/usr/lib, not /usr/lib/.debug .

Approved by: re (gjb)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation