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>

arch/blackfin/mach-bf533/boards/stamp.c

@@ -17,6 +17,7 @@
 #if IS_ENABLED(CONFIG_USB_ISP1362_HCD)
 #include <linux/usb/isp1362.h>
 #endif
+#include <linux/gpio.h>
 #include <linux/irq.h>
 #include <linux/i2c.h>
 #include <asm/dma.h>