scanadf: added missing call to sane_exit()

Failure to call sane_exit() which causes issues in genesys backend
termination.

src/scanadf.c

@@ -1709,6 +1709,7 @@ List of available devices:", prog_name);
 
   sane_cancel (device);
   sane_close (device);
+  sane_exit();
 
   if (scriptWait)
     while (wait(NULL) != -1);