package/rng-tools: add nistbeacon option

Add an option to enable nistbeacon which is available since version 6.1
and
https://github.com/nhorman/rng-tools/commit/aefe862e187a1ef608f9826b2b4a5bbabd5e9945



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>