Makefile.lib: include /__symbols__ in dtb if SPL_LOAD_FIT_APPLY_OVERLAY is enabled

In order to apply an overlay to a DTB. The DTB must have been generated
with the option '-@'.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>