ruby: really disable ncurses/readline for host

Commit 150124b9 (ruby: fix host build breakage) tried to fix host-ruby
build failures by disabling ncurses/readline extensions, but the
ruby configure script doesn't understand --without-ncurses /
--without-readline arguments, so the commit didn't do anything.

Fix it by really disabling them using the --with-out-ext argument.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>