FreeBSD: Add const qualifier to members of struct opensolaris_utsname

These members have directly references to the global variables
exposed by the kernel. They are not going to be changed by this
kernel module.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Zhenlei Huang <zlei@FreeBSD.org>
Closes #16210