pgsql: Define the value for in6addr_any on MingW, since it provides the

Log Message:
-----------
Define the value for in6addr_any on MingW, since it provides the struct
only in the header files and not in any libraries, yet declare it as
an extern.

Modified Files:
--------------
    pgsql/src/backend/port/win32:
        mingwcompat.c (r1.8 -> r1.9)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/mingwcompat.c?r1=1.8&r2=1.9)