mraa: add deinit function for valgrind testing

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

api/mraa/common.h

@@ -186,6 +186,14 @@ mraa_result_t mraa_init() __attribute__((constructor));
 mraa_result_t mraa_init();
 #endif
 
+/**
+ * De-Initilise MRAA
+ *
+ * This is not a strict requirement but useful to test memory leaks and for
+ * people who like super clean code.
+ */
+void mraa_deinit();
+
 /**
  * Checks if a pin is able to use the passed in mode.
  *