avr32: remove duplicated #include

Remove duplicated #include in arch/avr32/boards/hammerhead/flash.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Acked-by: Alex Raimondi <mailinglist@miromico.ch>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>