Fix const-generic warning in xp_ssl.c

```
ext/openssl/xp_ssl.c:423:24: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  423 |         if (!(wildcard = strchr(certname, '*')) || memchr(certname, '.', wildcard - certname)) {
      |                        ^
```

Since these typically turn into errors with -Werror, fix the build
already.

Author: ndossche

ext/openssl/xp_ssl.c

@@ -413,7 +413,7 @@ static bool php_openssl_x509_fingerprint_match(X509 *peer, zval *val)
 
 static bool php_openssl_matches_wildcard_name(const char *subjectname, const char *certname) /* {{{ */
 {
-	char *wildcard = NULL;
+	const char *wildcard = NULL;
 	ptrdiff_t prefix_len;
 	size_t suffix_len, subject_len;