added #include<unistd.h> in udp_engine.cpp to solve error 'close was not declared in scope'

src/udp_engine.cpp

@@ -4,6 +4,7 @@
 #include "windows.hpp"
 #else
 #include <sys/types.h>
+#include <unistd.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>