Add X509_STORE_CTX_get_num_untrusted

[h-vn]

X509_STORE_CTX_get_num_untrusted() was added to OpenSSL 1.1.0.

For more information, see:

• https://www.openssl.org/docs/manmaster/man3/X509_STORE_CTX_get_num_untrusted.html
• https://github.com/openssl/openssl/commit/7f3f41d816bb80e362a5978420f59030b3132c81