tools: mkimage: set OPENSSL_API_COMPAT

Building with OpenSSL 3.0 produces warnings like:

../tools/sunxi_toc0.c:846:17: warning: ‘RSA_get0_d’ is deprecated:
Since OpenSSL 3.0 [-Wdeprecated-declarations]
  846 |                 if (root_key && RSA_get0_d(root_key)) {
      |                 ^~

As OpenSSL 3.0 is not available in elder Linux distributions
just silence the warning.

Add missing #include <openssl/bn.h>.

Fixes: e9e87ec4 ("tools: mkimage: Add Allwinner TOC0 support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>