getentropy(3): sort SEE ALSO sections

Sorting order should be done by manpage section (2 vs 3), then alphabetically.
This change fixes the order to sort by the manpage section, first.

Reported by: make manlint
MFC after: 1 week
Sponsored by: DellEMC Isilon

(cherry picked from commit eca1933a55ae8ca916f75ed1ff8f12789e54db8a)