rtl8187: Fix sparse warnings

Due to a missing header include, sparse generates the following warnings:

  CHECK   drivers/net/wireless/rtl818x/rtl8187_rfkill.c
warning: symbol 'rtl8187_rfkill_init' was not declared. Should it be static?
warning: symbol 'rtl8187_rfkill_poll' was not declared. Should it be static?
warning: symbol 'rtl8187_rfkill_exit' was not declared. Should it be static?

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>