openSUSE Tumbleweed attempts to eliminate the need for libncurses.so.5 and
getting all packages in openSUSE ncurses6 compatible.

The list is getting rather short already - one of the last consumers of this
library is 'samba-clients'

A quick look into the buildsystem of samba shows that it calls explicitly
'ncurses5-config' to find the required libraries and cflags. There is a very
good chance that simply switching this to ncurses6-config would be sufficient
for the switch

In any case, ncurses5 won't be provided by ncurses-devel by default... so if a
switch is not possible, a bare minimum would be to change the BuildRequires to
ncurses5-devel