Make ieee80211_add_ssid() public.

Some drivers use private copies of this.

PR:		kern/196116
Submitted by:	Andriy Voskoboinyk <s3erios@gmail.com>


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275928 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f