main/iostream.c: fix build with libressl

Fix the following build failure with libressl by using SSL_is_server
which is available since version 2.7.0 and
d7ec516916:

iostream.c: In function 'ast_iostream_close':
iostream.c:559:41: error: invalid use of incomplete typedef 'SSL' {aka 'struct ssl_st'}
  559 |                         if (!stream->ssl->server) {
      |                                         ^~

ASTERISK-30107 #close

Fixes: - http://autobuild.buildroot.org/results/ce4d62d00bb77ba5b303cacf6be7e350581a62f9
Change-Id: Iea7f34970297f2fb50285d73462d0174ba7e9587
(cherry picked from commit 030b7b9009)

main/iostream.c

@@ -553,7 +553,7 @@ int ast_iostream_close(struct ast_iostream *stream)
 					ERR_error_string(sslerr, err), ssl_error_to_string(sslerr, res));
 			}
 
-#if !defined(LIBRESSL_VERSION_NUMBER) && (OPENSSL_VERSION_NUMBER >= 0x10100000L)
+#if !(defined(LIBRESSL_VERSION_NUMBER) && (LIBRESSL_VERSION_NUMBER < 0x2070000L)) && (OPENSSL_VERSION_NUMBER >= 0x10100000L)
 			if (!SSL_is_server(stream->ssl)) {
 #else
 			if (!stream->ssl->server) {