Fixed compilation warning of no return on non-void method.

[SumolX]

Fixed warning:

lib/embeddedRTPS/include/rtps/entities/StatefulReader.tpp: In member function 'bool rtps::StatefulReaderT<NetworkDriver>::onNewGapMessage(const rtps::SubmessageGap&, const rtps::GuidPrefix_t&) [with NetworkDriver = rtps::UdpDriver]': lib/embeddedRTPS/include/rtps/entities/StatefulReader.tpp:115:8: warning: control reaches end of non-void function [-Wreturn-type]