security/nebula: fix build on 32 bit platforms

The structure syscall.Iovec has 32 bit fields on 32 bit platforms,
64 bit fields on 64 bit platforms. Go copes poorly with this difference
(why didn't they just use int?). Apply a conditional patch to fix the
build on 32 bit platforms.

Approved by: portmgr (build fix blanket)
MFH: 2026Q2