Added conditional to prevent redeclaration of class

Some users have reported that using APC and/or FastCGI causes an
internal server error with the declaration of the class used to manage
the code for the malware scanner settings; this is because APC has a bug
that duplicates the autoloaders thinking that a class was defined twice,
the only apparent solution is to set these settings:

apc.include_once_override = 0
apc.canonicalize = 0
apc.stat = 0

sucuri.php

@@ -13555,6 +13555,7 @@ function sucuriscan_settings_corefiles_cache($nonce)
     return SucuriScanTemplate::getSection('settings-corefiles-cache', $params);
 }
 
+if (!class_exists('SucuriScanSiteCheck')) {
 class SucuriScanSiteCheck extends SucuriScanSettings
 {
     public static function isEnabled()
@@ -13647,6 +13648,7 @@ public static function timeoutPage($nonce)
         return SucuriScanTemplate::getSection('settings-sitecheck-timeout', $params);
     }
 }
+}
 
 /**
  * Read and parse the content of the SiteCheck settings template.