fix a compile time error

[ghost]

the demo code:

import ncurses

initscr()
printw("hello world")
refresh()
getch()
endwin()

error message:

~/nim-ncurses/ncurses.nim(237, 32) template/generic instantiation of `NCURSES_BITS` from here
~/nim-ncurses/ncurses.nim(84, 16) Error: -1 can't be converted to chtype

define chtype with cint will fix it.