staging: rtl8192u: ieee80211: Converted symbol to static.

This patch fixes this sparse warning:
drivers/staging/rtl8192u/ieee80211/ieee80211_crypt_ccmp.c:60:6: warning:
symbol 'ieee80211_ccmp_aes_encrypt' was not declared. Should it be static?

Signed-off-by: Aybuke Ozdemir <aybuke.147@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>