#10 Fix build against ncurses-6.5

[Sergei Trofimovich]

Without the change build fails on ncurses-6.5 (enables opaque WINDOW by
default) as:

aacurses.c: In function 'curses_getsize':
aacurses.c:74:20: error: invalid use of incomplete typedef 'WINDOW' {aka 'struct _win_st'}
 74 |     *width = stdscr->_maxx + 1;
    |                    ^~