kern_linker.c: sort includes

This is preferred by style(9). Do this ahead of adding another include.

Reviewed by: imp, kevans
MFC after: 3 days
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D30185

(cherry picked from commit 877eea429ba5bbe7d28aca26d31fe15e8e14f8e2)