libdtrace: Explicitly set SHLIBDIR and SHLIB_MAJOR

They were previously being defined by cddl/lib/Makefile.inc, and as of
commit 4ae699122810 were being overridden by defaults in bsd.own.mk,
which changed SHLIBDIR to /usr/lib.

Reported by: Domagoj Stolfa <ds815@cam.ac.uk>
Fixes: 4ae699122810 ("dtrace: Add WITH_DTRACE_ASAN")