mraa_internal.h: Added mraa_iio_detect() function declaration

Signed-off-by: Henry Bruce <henry.bruce@intel.com>

include/mraa_internal.h

@@ -67,6 +67,13 @@ mraa_platform_t mraa_arm_platform();
  */
 mraa_platform_t mraa_usb_platform_extender(mraa_board_t* board);
 
+/**
+ * runtime detect iio subsystem
+ *
+ * @return mraa_result_t indicating success of iio detection
+ */
+mraa_result_t mraa_iio_detect();
+
 /**
  * helper function to check if file exists
  *