common.h: Added destructor attribute for deinit
Currently, mraa_init is called when loading libmraa.  This commit
adds similar functionality to call mraa_deinit when unloading
libmraa.  This is a nice-to-have since it makes identifying other
memory leaks easier.
@@ -85,7 +85,7 @@ mraa_result_t mraa_init() __attribute__((constructor));
* people who like super clean code. If dynamically loading & unloading
* libmraa you need to call this before unloading the library.
*/
-void mraa_deinit();
+void mraa_deinit() __attribute__((destructor));
/**
* Checks if a pin is able to use the passed in mode.
