net80211: MFC _debrefcnt_used

While 4a8e4d15465cbf6f1bc4c6a09f50f97ae2d5bd7d (fix IEEE80211_DEBUG_REFCNT
builds) covers more parts, we cannot MFC all due to KPI/KBI stability.
We only pick the conditional define of __debrefcnt_used in order to allow
commits following the original one to be MFCed more easily.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 4a8e4d15465cbf6f1bc4c6a09f50f97ae2d5bd7d)