fix invalid conversion error with MinGW 4.8.1 in net.cpp

- fixes src\net.cpp:1601: Error:invalid conversion from 'void*' to
  'const char*' [-fpermissive] in a setsockopt() call on Win32 that was
  found by using MinGW 4.8.1 compiler suite

src/net.cpp

@@ -1732,8 +1732,12 @@ bool BindListenPort(const CService &addrBind, string& strError)
     // and enable it by default or not. Try to enable it, if possible.
     if (addrBind.IsIPv6()) {
 #ifdef IPV6_V6ONLY
+#ifdef WIN32
+        setsockopt(hListenSocket, IPPROTO_IPV6, IPV6_V6ONLY, (const char*)&nOne, sizeof(int));
+#else
         setsockopt(hListenSocket, IPPROTO_IPV6, IPV6_V6ONLY, (void*)&nOne, sizeof(int));
 #endif
+#endif
 #ifdef WIN32
         int nProtLevel = 10 /* PROTECTION_LEVEL_UNRESTRICTED */;
         int nParameterId = 23 /* IPV6_PROTECTION_LEVEl */;