LinuxKPI: fix build

Add a missing "static" for non-{i386,amd64,arm64} which was missed in
c39eefe715b3c835ce3d91a1c1932197c23c1f3c. This should ifx the builds.

Sponsored by: The FreeBSD Foundation
MFC after: 7 days
X-MFC with: c39eefe715b3c835ce3d91a1c1932197c23c1f3c