curses: fix compilation errors when host has no curses library.

* ext/posix/curses/chstr.c: Adjust #ifdef HAVE_CURSES to make
sure there's an emptyg posix_curses_chstr_fns definition.
* ext/posix/curses/window.c: Likewise for
posix_curses_windows_fns.
* ext/posix/curses.h: Likewise for curseslib.
Also move #includes for the above files out of the HAVE_CURSES
guard.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>