Move back the PHP 5.4 improvements file to /inc

The problem with the autoload for the 5.4 file is that the Syntax Error is triggered at the moment of loading/reading the file - before executing any of it's code. Therefore the only way to load the 5.4 file without breaking the entire WordPress install due to a PHP parsing error is verifying the PHP version before the import.

That's why my proposal is moving it back to inc/ to prevent fatal errors.