[BUGFIX] BE ajax request fails on some systems with php 5.3

[BUGFIX] BE ajax request fails on some systems with php 5.3

Because of a PHP bug on some systems the request to ajax.php fails because it
includes '../t3lib/class.t3lib_div.php' with relative path while init.php uses
PATH_t3lib . 'class.t3lib_div.php'.
On some systems PHP is not able to detect the already included file when a
different path is used.

Change-Id: I66e39690747c4f78b4571cf63777d98cb88b6b75
Resolves: #M17250
Reviewed-on: http://review.typo3.org/1286
Reviewed-by: Joh. Feustel
Tested-by: Joh. Feustel
Reviewed-by: Steffen Kamper
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers