samba4: specify ncurses-config

When ncurses wide is enabled samba doesn't automatically find the
appropiate ncurses-config script and finds the host variant (which is
non-widec) which leaks improper library directories into the build.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>