$cache_enable should be defined as a global var in SSI

Fixes SimpleMachines#5061

Signed-off-by: John Rayes <live627@gmail.com>

SSI.php

@@ -22,7 +22,7 @@
 global $boardurl, $boarddir, $sourcedir, $webmaster_email, $cookiename;
 global $db_type, $db_server, $db_name, $db_user, $db_prefix, $db_persist, $db_error_send, $db_last_error;
 global $db_connection, $db_port, $modSettings, $context, $sc, $user_info, $topic, $board, $txt;
-global $smcFunc, $ssi_db_user, $scripturl, $ssi_db_passwd, $db_passwd, $cachedir;
+global $smcFunc, $ssi_db_user, $scripturl, $ssi_db_passwd, $db_passwd, $cache_enable, $cachedir;
 
 // Remember the current configuration so it can be set back.
 $time_start = microtime(true);