standardize ns-3 namespace declaration format

src/core/model/breakpoint.cc

src/core/model/breakpoint.h

src/core/model/event-id.cc

src/core/model/event-id.h

src/core/model/names.cc

src/core/model/names.h

src/core/model/ptr.h

src/core/model/realtime-simulator-impl.cc

src/core/model/rng-stream.cc

src/core/model/rng-stream.h

src/core/model/simulation-singleton.h

src/core/model/synchronizer.h

src/core/model/system-condition.h

src/core/model/system-mutex.h

src/core/model/system-thread.h

src/core/model/system-wall-clock-ms.h

src/core/model/traced-callback.h

src/core/model/type-name.cc

src/core/model/type-name.h

src/core/model/unix-system-wall-clock-ms.cc