compile_et: reduce the dependency chain for bootstrap tools.

Fix the build on macOS

The bootstrap tools are statically linked, so our build system
will provide the full dependency chain which pulls in libcrypt.

since recently libcrypt.a is a linker script not supported by macOS
ar(1), given that compile_et does not use at all the function from
libroken which brings the dependency on libcrypt, bypass LIBADD
dependency chain by using the old LDADD/DPADD mechanism, like it is
done for other kerberos related bootstrap tools.