uorb: fix constness for _uorb_topics_list

_uorb_topics_list was marked as 'const char *' array, which means the data
of the array was not actually const and thus landed in the data section
(so in RAM instead of FLASH).
The size of the array is 436 bytes.

src/modules/uORB/uORBTopics.h

@@ -45,6 +45,6 @@ extern size_t orb_topics_count() __EXPORT;
 /*
  * Returns array of topics metadata
  */
-extern const struct orb_metadata **orb_get_topics() __EXPORT;
+extern const struct orb_metadata *const *orb_get_topics() __EXPORT;
 
 #endif /* MODULES_UORB_UORBTOPICS_H_ */