tools: mkimage: fix build with recent LibreSSL

LibreSSL 3.5.0 and later (also shipped as part of OpenBSD 7.1 and
and later) have an opaque RSA object and do provide the
RSA_get0_* functions that OpenSSL provides.

Fixes: 2ecc354b ("tools: mkimage: fix build with LibreSSL")
Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Jonathan Gray <jsg@jsg.id.au>