icmp: hide icmp_bandlimit_uninit() under VIMAGE

The uninitialization may be executed only on a kernel with VIMAGE.

Reviewed by: kp, tuexen, zlei
Differential Revision: https://reviews.freebsd.org/D44476
Approved by: re (cperciva)

(cherry picked from commit 9d7f17d7467ed8c9740730a8db7a82e4768e5177)
(cherry picked from commit a6d5c6a04fe9408cdfd1f56f8dc3142661214eb8)