Fix windows.h being included before winsock.h

ethminer/main.cpp

@@ -20,6 +20,14 @@
  * Ethereum client.
  */
 
+// Solves the problem of including windows.h before including winsock.h
+// as detailed here:
+// http://stackoverflow.com/questions/1372480/c-redefinition-header-files-winsock2-h
+#if defined(_WIN32)
+#define _WINSOCKAPI_
+#include <windows.h>
+#endif
+
 #include <thread>
 #include <chrono>
 #include <fstream>