mac80211: make rate_control_alloc static

rate_control_alloc is not used by anything outside of
ieee80211_init_rate_ctrl_alg.  Both are in rate.c; there's no reason to make
rate_control_alloc visible outside of it.

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Signed-off-by: John W. Linville <linville@tuxdriver.com>