DDSPSMC11_ — API correction required to dds/sub/find.hpp

The return type of dds::sub::builtin_subscriber() is a const reference, forcing to keep the builtin subscriber as a member in the DomainParticipant.

We cannot make the subscriber a member of the DomainParticipant because this would cause a shared_ptr loop. This also unnecessarily restricts the implementation of this function.

Proposed resolution:

- const dds::sub::Subscriber&
- builtin_subscriber(const dds::domain::DomainParticipant& dp);
+ dds::sub::Subscriber 
+ builtin_subscriber(const dds::domain::DomainParticipant& dp);