Make the discovery example depend on UA_ENABLE_DISCOVERY_MULTICAST
Make the discovery example depend on UA_ENABLE_DISCOVERY_MULTICAST

Without having open62541 configured with UA_ENABLE_DISCOVERY_MULTICAST the examples
are not functional.

./discovery_server_register exits early with an error.
./discovery_server_lds complains on connect of ./discovery_server_register about:
info/channel Connection 6 | SecureChannel 3 | Unknown request with type identifier 12208

jpfr/open62541,JGrothoff/open62541,StalderT/open62541,open62541/open62541,JGrothoff/open62541,jpfr/open62541,StalderT/open62541,jpfr/open62541,jpfr/open62541,StalderT/open62541,open62541/open62541,open62541/open62541,open62541/open62541,StalderT/open62541,JGrothoff/open62541,JGrothoff/open62541