mx23_olinuxino: Fix warning for implicit declaration

Fixes a build warning of implicit declaration of
gpio_direction_output, as bellow:

,----
| mx23_olinuxino.c: In function 'board_early_init_f':
| mx23_olinuxino.c:51:2: warning: implicit declaration
|     of function 'gpio_direction_output'
|     [-Wimplicit-function-declaration]
`----

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
diff --git a/board/olimex/mx23_olinuxino/mx23_olinuxino.c b/board/olimex/mx23_olinuxino/mx23_olinuxino.c
index 9252717..9ed7718 100644
--- a/board/olimex/mx23_olinuxino/mx23_olinuxino.c
+++ b/board/olimex/mx23_olinuxino/mx23_olinuxino.c
@@ -23,6 +23,7 @@
  */
 
 #include <common.h>
+#include <asm/gpio.h>
 #include <asm/io.h>
 #include <asm/arch/iomux-mx23.h>
 #include <asm/arch/imx-regs.h>