build: do not generate support for libuv's probes

Dtrace probes were removed from libuv recently, but their usage by node
was not completely removed, causing build breaks on SmartOS.

Even though the build is working on other platforms, these probes are
not fired by libuv anymore, so there's no point in using them on these
platforms too.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>