Minimize the contents of the INTERFACE_INCLUDE_DIRECTORIES

The property only needs to contain the direct include dirs of
a target. For example, Qt5::Gui does not need to contain the
include/QtCore directory because it already has Qt5::Core in
its LINK_INTERFACE_LIBRARIES.

Change-Id: I69612f42c29e6056b3d15399498d041d43a0dd6b
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>