ERR_load_BIO_strings' is deprecated

[coterobarros]

Thanks for this piece of code, so good! However, I've found some deprecation warnings when compiling on OSX with openssl 3.0.5, installed with brew:

gcc -DSMTP_OPENSSL -I/usr/local/Cellar/openssl@3/3.0.5/include smtp/smtp.c -c -o smtp.o
smtp/smtp.c:1757:3: warning: 'ERR_load_BIO_strings' is deprecated [-Wdeprecated-declarations]
  ERR_load_BIO_strings();
  ^
/usr/local/Cellar/openssl@3/3.0.5/include/openssl/cryptoerr_legacy.h:31:1: note: 'ERR_load_BIO_strings' has been explicitly marked deprecated here
OSSL_DEPRECATEDIN_3_0 int ERR_load_BIO_strings(void);
^
/usr/local/Cellar/openssl@3/3.0.5/include/openssl/macros.h:182:49: note: expanded from macro 'OSSL_DEPRECATEDIN_3_0'
#   define OSSL_DEPRECATEDIN_3_0                OSSL_DEPRECATED(3.0)
                                                ^
/usr/local/Cellar/openssl@3/3.0.5/include/openssl/macros.h:62:52: note: expanded from macro 'OSSL_DEPRECATED'
#     define OSSL_DEPRECATED(since) __attribute__((deprecated))
                                                   ^
1 warning generated.