ncurses: mark upstream version as unknown

This is to avoid check-version reporting a broken current version
(6.3+nnnn sorts higher than 6.3 and isn't tagged in upstream git).

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>