Replace fallthrough comments with G_GNUC_FALLTHROUGH

It's safer to do it this way and since we have G_GNUC_FALLTHROUGH now, w
e might as well replace the fallthrough comments.

glib/gshell.c

@@ -523,7 +523,7 @@ tokenize_command_line (const gchar *command_line,
               ensure_token (&current_token);
               g_string_append_c (current_token, *p);
 
-              /* FALL THRU */
+              G_GNUC_FALLTHROUGH;
             case '\\':
               current_quote = *p;
               break;