sysutils/osquery: unbreak with boost 1.69

In file included from osquery/remote/serializers/json.cpp:11:
In file included from osquery/core/json.h:15:
In file included from include/osquery/system.h:23:
include/osquery/posix/system.h:29:39: error: expected class name
class DropPrivileges : private boost::noncopyable {

^

In file included from osquery/remote/serializers/json.cpp:11:
In file included from osquery/core/json.h:15:
include/osquery/system.h:42:36: error: expected class name
class Initializer : private boost::noncopyable {

^

PR: 232525