imgact: Mark brandinfo and note structures as const

No functional change intended.

Reviewed by: olce, kib, emaste
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D53062

(cherry picked from commit 80336636b6b9f7a3bdad007c400e85eae017d2a2)

imgact: Mark brandinfo and note structures as const

No functional change intended.

Reviewed by: kib
MFC after: 10 days
MFC with: 80336636b6b9f7a3bdad007c400e85eae017d2a2
Differential Revision: https://reviews.freebsd.org/D53173

(cherry picked from commit 301e6c47127d358073a35c1ecd7b8cf014d0bb92)