daybreak: fix strncpy compiler warning

GCC warned that the output string would not be nul-terminated if the
input string was too long.  Fix this by subtracting 1 from the size
argument, as is done for other `strncpy` calls in the file.  (It would
probably be better to avoid `strncpy` entirely, but this is just a
simple fix.)

troposphere/daybreak/source/ui.cpp

@@ -1277,7 +1277,7 @@ namespace dbk {
         if (InitializeMenu(screen_width, screen_height)) {
 
             /* Set the update path. */
-            strncpy(g_update_path, update_path, sizeof(g_update_path));
+            strncpy(g_update_path, update_path, sizeof(g_update_path)-1);
 
             /* Change the menu. */
             ChangeMenu(std::make_shared<ValidateUpdateMenu>(g_current_menu));