minui: Add missing <linux/ion.h> for 'struct ion_handle_data'

Change-Id: I87133cc8409ab988bbffa40d790e138c25b01309
diff --git a/minui/graphics_overlay.h b/minui/graphics_overlay.h
index 56f7401..1b2f19a 100644
--- a/minui/graphics_overlay.h
+++ b/minui/graphics_overlay.h
@@ -18,6 +18,7 @@
 #define _GRAPHICS_OVERLAY_H_
 
 #include <linux/fb.h>
+#include <linux/ion.h>
 
 #include "graphics.h"
 #include "minui/minui.h"