getopt_long: Fix broken sequence of casts in __UNCONST macor.

Unlike the compilation mode used for OVS x64 Linux Windows long word is
4 bytes for both 32 and 64 bit builds.
Replaced _UNCONST macro with CONST_CAST to avoid the intermediate casting
to an integer.

Testing: 32 and 64 Windows builds.
Signed-off-by: Eitan Eliahu eliahue@vmware.com
Signed-off-by: Ben Pfaff <blp@nicira.com>