mraa: add deinit function for valgrind testing

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

src/mraa.c

@@ -98,6 +98,13 @@ mraa_init()
     return MRAA_SUCCESS;
 }
 
+void
+mraa_deinit()
+{
+    free(plat->pins);
+    free(plat);
+}
+
 int
 mraa_set_priority(const unsigned int priority)
 {