Placed dependancy on BOARD_USES_PX4IO not path (PX4IO_DEVICE_PATH)

Code was conditionaly included based on #define PX4IO_DEVICE_PATH
    in drivers/drv_gpio.h depanedent on board #ifdef CONFIG_ARCH_BOARDxxxx
    Now that dependancy comes from board_common.h as BOARD_USES_PX4IO defined
    when a board config provides PX4IO_SERIAL_DEVICE.

src/systemcmds/tests/test_gpio.c

@@ -91,7 +91,7 @@ int test_gpio(int argc, char *argv[])
 {
 	int		ret = 0;
 
-#ifdef PX4IO_DEVICE_PATH
+#if defined(BOARD_USES_PX4IO)
 
 	int fd = px4_open(PX4IO_DEVICE_PATH, 0);