ftdi-gpio: fix -Wimplicit-function-declaration

Fixes:

  ftdi-gpio.c: In function ‘ftdi_gpio_open’:
  ftdi-gpio.c:190:9: warning: implicit declaration of function ‘usleep’ [-Wimplicit-function-declaration]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

ftdi-gpio.c

@@ -38,6 +38,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "cdba-server.h"
 #include "ftdi-gpio.h"