cpp: Add ServerThread as a friend class of Server.
cpp: Add ServerThread as a friend class of Server.

Seems to me that this should not be necessary since lo::ServerThread
inherits from lo::Server, but it fixes a compile error with g++ 4.8.1.

island-org/liblo,johne53/MB3Liblo,fsateler/liblo,fsateler/liblo,tuscland/liblo,umlaeute/liblo,tuscland/liblo,johne53/MB3Liblo,johne53/MB3Liblo,radarsat1/liblo,island-org/liblo,tuscland/liblo,umlaeute/liblo,kneitinger/liblo,island-org/liblo,johne53/MB3Liblo,radarsat1/liblo,radarsat1/liblo,umlaeute/liblo,kneitinger/liblo,kneitinger/liblo,fsateler/liblo