bf533: fix build error: add linux/gpio.h

build error
arch/blackfin/mach-bf533/boards/stamp.c: In function ‘stamp_init’:
arch/blackfin/mach-bf533/boards/stamp.c:866: error: implicit declaration of function ‘gpio_request’
arch/blackfin/mach-bf533/boards/stamp.c:868: error: implicit declaration of function ‘gpio_direction_output’
arch/blackfin/mach-bf533/boards/stamp.c:869: error: implicit declaration of function ‘gpio_free’

Signed-off-by: Steven Miao <realmz6@gmail.com>