Add dependency on the process library

The tests are already using this library, so it's not really a new build
dependency, but it was not specified esplicitly.

Furthermore, it's going to be used by the instance status collector, so it's
added to the requirements for the monitoring subsystem.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>