emulators/open-vm-tools: change sysctl_add_oid use to SYSCTL_ADD_OID

sysctl_add_oid was changed in base/head@r310051 to take a label parameter,
and open-vm-tools doesn't supply it as it depends on the old KPI signature.
SYSCTL_ADD_OID doesn't require the label parameter since its inception
though, thus it's a backwards compatible solution for adding sysctl oids.

PR: 215353
Reported by: dim
Submitted by: ed
Reviewed by: ed, lwhsu
Approved by: lwhsu
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D9698