cfg80211: make ieee80211_get_mesh_hdrlen() static

Fixes spares warning:
net/wireless/util.c:261:5: warning:
symbol 'ieee80211_get_mesh_hdrlen' was not declared. Should it be static?

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>