Fix build warning on LibreSSL

[ndossche]

When trying to build on LibreSSL, I encounter the following build warning (this was observed on master, but applies to 8.4 too):

/work/php-src/ext/openssl/openssl.c: In function 'zif_openssl_x509_parse':
/work/php-src/ext/openssl/openssl.c:1101:22: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1101 |                 purp = X509_PURPOSE_get0(i);
      |                      ^
/work/php-src/ext/openssl/openssl.c:1110:23: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
 1110 |                 pname = useshortnames ? X509_PURPOSE_get0_sname(purp) : X509_PURPOSE_get0_name(purp);