This is not distributed via Software Update, but via Apple's XProtect malware protection system and should be updated silently in the background. You can check

This is not distributed via Software Update, but via Apple's XProtect malware protection system and should be updated silently in the background. You can check /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta.plist to see if it's up to date. To force an update, run "sudo /usr/libexec/XProtectUpdater" or "sudo launchctl start com.apple.xprotectupdater".