windows: suppress a warning

    groonga.c: In function 'h_output':
    groonga.c:696:9: warning: passing argument 4 of 'WSASend' from incompatible pointer type
         if (WSASend(fd, wsabufs, 4, &ret, 0, NULL, NULL) == SOCKET_ERROR) {
             ^
    In file included from ../lib/grn.h:103:0,
                     from groonga.c:22:
    /usr/share/mingw-w64/include/winsock2.h:1048:34: note: expected 'LPDWORD' but argument is of type 'ssize_t *'
       WINSOCK_API_LINKAGE int WSAAPI WSASend(SOCKET s,LPWSABUF lpBuffers,DWORD dwBufferCount,LPDWORD lpNumberOfBytesSent,DWORD dwFlags,LPWSAOVERLAPPED lpOverlapped,LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine);
                                      ^