games/DDNet: fix build on GCC architectures

/wrkdirs/usr/ports/games/DDNet/work/DDNet-15.2.3/src/engine/server/server.cpp: In member function 'int CServer::Init()':
/wrkdirs/usr/ports/games/DDNet/work/DDNet-15.2.3/src/engine/server/server.cpp:525:2: error: 'memset' was not declared in this scope

525 |  memset(m_aPrevStates, CClient::STATE_EMPTY, MAX_CLIENTS * sizeof(int));