iflib: Make iflib_stop() static

It is declared as static. Make the definition consistent with the
declaration.

This follows 7ff9ae90f0bd and partially reverts 09f6ff4f1a47.

Reviewed by: erj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46185

(cherry picked from commit 8161000892830ee52bc8048be91b40cdad25fea8)