Add OpenSSL 3.0 support

[Peter Wienemann]

Building ssldump version 1.8 using OpenSSL 3.0 results in a number of deprecation warnings of the following kind:

/<<PKGBUILDDIR>>/ssl/ssldecode.c: In function ‘ssl_process_client_key_exchange’:
/<<PKGBUILDDIR>>/ssl/ssldecode.c:670:7: warning: ‘RSA_get0_key’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
  670 |       RSA_get0_key(EVP_PKEY_get0_RSA(pk), &n, NULL, NULL);
      |       ^~~~~~~~~~~~
In file included from /usr/include/openssl/x509.h:36,
                 from /usr/include/openssl/ssl.h:31,
                 from /<<PKGBUILDDIR>>/ssl/ssldecode.c:53:
/usr/include/openssl/rsa.h:217:28: note: declared here