tools: -Wno-deprecated-declarations for OpenSSL on darwin

Since OpenSSL is deprecated on OS X in favour of Common Crypto API disable the
warning for this host OS.

Another solution would be to add some glue layer for crypto stuff, but I think
this is not worth the effort.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>