util: Make WSAStartup available outside stream.c.

WSAStartup() needs to be called before using winsock2 related
functions. We need this for almost all the utilities. So call
it through OVS_CONSTRUCTOR.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>