iwlwifi: iwl-agn.h included twice

drivers/net/wireless/iwlwifi/iwl-core.c included 'iwl-agn.h' twice,
remove the duplicate.

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>