diff3: Remove bespoke getopt.h

diff3's getopt.h included a function declaration without a prototype,
which produces a compiler warning. Just remove the bespoke getopt.h
and use the system header.

Reported by: Mark Millard
Reviewed by: fuz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53802

(cherry picked from commit 38829592dc77e9ead4259785e0bfb93bd4c2fe34)