WP_ADMIN needs to be true in flash uploader

[hailin]

<mdawaffe_lap_> eagleclaw: wp-admin/async-upload.php loads admin.php after wp-load.php, so WP_ADMIN is not defined by the time plugins are loaded.

It needs to be TRUE because some plugin tests this variable to make sure the URL is a admin one.

[hailin]

patch

[azaozz]

(In [10827]) Define WP_ADMIN in flash uploader, props hailin, fixes #9368