windows: suppress a warning

    com.c: In function 'grn_com_copen':
    com.c:960:16: warning: passing argument 4 of 'setsockopt' from incompatible pointer type
         } else if (setsockopt(fd, 6, TCP_NODELAY, &value, sizeof(value))) {
                    ^
    In file included from groonga_in.h:103:0,
                     from com.c:18:
    /usr/share/mingw-w64/include/winsock2.h:999:34: note: expected 'const char *' but argument is of type 'const int *'
       WINSOCK_API_LINKAGE int WSAAPI setsockopt(SOCKET s,int level,int optname,const char *optval,int optlen);
                                      ^