AP_HAL_Linux: Util: fix overriden function signature

While at it, add final and override to mark this as being the overriden
final implementation of this method.

Thanks to Phillip Khandeliants (@khandeliants) for reporting.

libraries/AP_HAL_Linux/Util.h

@@ -48,7 +48,7 @@ public:
      */
     void set_system_clock(uint64_t time_utc_usec);
     const char* get_custom_log_directory() { return custom_log_directory; }
-    const char* get_custom_terrain_directory() { return custom_terrain_directory; }
+    const char* get_custom_terrain_directory() const override final { return custom_terrain_directory; }
 
     void set_custom_log_directory(const char *_custom_log_directory) { custom_log_directory = _custom_log_directory; }
     void set_custom_terrain_directory(const char *_custom_terrain_directory) { custom_terrain_directory = _custom_terrain_directory; }