Correctly close socket on win32

Refs : https://msdn.microsoft.com/en-us/library/windows/desktop/ms740126%28v=vs.85%29.aspx

src/dhtrunner.cpp

@@ -37,6 +37,7 @@
 #else
 #include <winsock2.h>
 #include <ws2tcpip.h>
+#define close(x) closesocket(x)
 #endif
 
 namespace dht {