ncurses: enable parallel build

Using a trick taken from gentoo enable parallel build when building for
the target to reduce the build time for my test case from about 54s to
32s on a dual-core laptop.

[Peter: add comment explaining why]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>