deps: do not link against librt

It is not needed, and causes an unnecessary runtime dependency with some
linkers, such as devtoolset-6 on centos7-ppc64le.

See: https://github.com/nodejs/node/issues/29718

PR-URL: https://github.com/nodejs/node/pull/29729


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: David Carlier <devnexen@gmail.com>