Change PHP meterpreter header comment style

This commit fixes cases where stageless meterpreter payloads may not run
if they are loaded within a PHP context that's already inside the
opening and closing <?php ... ?> tags. While this is rare, it's possible
that this may happen. This approach matches that which we use for staged
payloads.

php/meterpreter/meterpreter.php

@@ -1,4 +1,4 @@
-//<?php
+/*<?php /**/
 
 # Everything that needs to be global has to be made so explicitly so we can run
 # inside a call to create_user_func($user_input);