Avoid implicit declaration of function addnwstr.

* pinentry/pinentry-curses.c [HAVE_NCURSESW]: Include <ncursesw/curses.h>.
[! HAVE_NCURSESW]: Only include <curses.h> in this case.

When built with libncursesw, we see this problem:

pinentry-curses.c:440:8: warning: implicit declaration of function ‘addnwstr’ [-Wimplicit-function-declaration]

ADDCH (start[i]);
^