Makefile: Silence the deprecation script

This bash code is currently showing up in the build unless 'make -s' is
used. We don't normally show these sorts of things as they are confusing.
Also this code was not shown before the recent refactoring of how these
messages are displayed.

Add an '@' to silence it.

Suggested-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Bin Meng <bmeng.cn@gmail.com>
Fixes: 13732528 ("Makefile: Add common code to report deprecation")