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

(cherry picked from commit 9d7f17d7467ed8c9740730a8db7a82e4768e5177)