Fix build with upcoming libressl release

The SSL_SESSION and SSL3_STATE structs will become opaque in the
upcoming LibreSSL version 3.4.x. This will break the build. The
requisite accessors have been available since LibreSSL 2.7.0, so
adjust the version check accordingly.

ssl.c

@@ -598,7 +598,7 @@ ssl_ssl_masterkey_to_str(SSL *ssl)
 	char *str = NULL;
 	int rv;
 	unsigned char *k, *r;
-#if (OPENSSL_VERSION_NUMBER >= 0x10100000L) && !defined(LIBRESSL_VERSION_NUMBER)
+#if (OPENSSL_VERSION_NUMBER >= 0x10100000L) && (!defined(LIBRESSL_VERSION_NUMBER) || LIBRESSL_VERSION_NUMBER >= 0x2070000fL)
 	unsigned char kbuf[48], rbuf[32];
 	k = &kbuf[0];
 	r = &rbuf[0];