MFC r340889

sfxge(4): fix warnings from VS2015 C compiler (C4189)

Fix multiple level 4 warnings
"C4189: 'xxx': local variable is initialized but not referenced";
no functional changes.

Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18123