mac80211: fix defined but not used

These two symbols are used only in ifdeffed function. Move them to that
section too.
net/mac80211/sta_info.c:387: warning: `__sta_info_pin' defined but not used
net/mac80211/sta_info.c:397: warning: `__sta_info_unpin' defined but not used

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Michael Wu <flamingice@sourmilk.net>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>