qdoc: excludedirs works again

It was comparing an absolute path to a relative path.
These could never be the same. Instead of the relative
path, it now gets the canonical absolute path, so the
comparison succeeds when it should.

Task-number: QTBUG-31404
Change-Id: I9c482d4649d493ce6d4f9a522cb61f2c8a5eb21f
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>