dc240: fix [-Wunused-parameter] compiler warning

The attribute needs to go before the pointer designation for it to have
any effect.

backend/dc240.c

@@ -1643,7 +1643,7 @@ sane_set_io_mode (SANE_Handle UNUSEDARG handle, SANE_Bool
 }
 
 SANE_Status
-sane_get_select_fd (SANE_Handle UNUSEDARG handle, SANE_Int * UNUSEDARG fd)
+sane_get_select_fd (SANE_Handle UNUSEDARG handle, SANE_Int UNUSEDARG * fd)
 {
   return SANE_STATUS_UNSUPPORTED;
 }