libbe(3): Fix mis-application of patch (SHLIBDIR)

Rob's patch in D18564 cemented the SHLIBDIR because bsd.own.mk (included by
src.opts.mk) sets it to /usr/lib. r346546 did somehow not apply this part of
the patch, leaving it to get installed to the wrong place and subsequently
removed via ObsoleteFiles.

Reported by: jkim
MFC after: 3 days
X-MFC-With: r346546