init_main.c: sort includes

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

Reviewed by: imp, kevans, allanjude
MFC after: 3 days
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D30186

(cherry picked from commit 0aa9ffcd9c3ab174aeece3e8af611e796911a5d1)