ncursesw: add type alias for wint_t

On the C side, `wint_t' is a typedef used by
many of the wide char curses operations.
Previously, the binding assumed that `wint_t' is
an unsigned int.
Issue #53 indicates that the assumption about the
underlying type of `wint_t' may not hold
(the user experiences a build failure because
`CInt' is provided where `CUInt' is expected).

Perusal of curses.h indicates that `wint_t'
should be unsigned int or not defined at all.
Nevertheless, to work around the issue, this patch
introduces a corresponding type alias `Wint_t' on
the Haskell side which is set to the appropriate
type by c2hs at compile time.