[PATCH] hostap: Make hostap_tx_encrypt() static

hostap_tx_encrypt() is used only inside hostap_80211_tx.c and there
are no plans to use it elsewhere in the future either, so let's make
it static. As a bonus, this should silence Coverity scanner from
complaining about bogus FORWARD_NULL case (CID: 274).

Signed-off-by: Jouni Malinen <jkmaline@cc.hut.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>