disable module when accessibility is not available

Using requires() instead of error() makes the build finish
gracefully when accessibility is not enabled.

This is useful when trying to compile the full Qt5 on platforms
like WinCE.

Change-Id: I4a0f6338c0bd406941b34f4d285b0577f3130dd6
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>