module: use the header in RTT_ROOT if possible

There is no guarantee that the headers within
software/programs/rt-thread is always up-to-date. So use the header in
RTT_ROOT if possible. It will make use of RTT_RTGUI as well.
Merge pull request #15 from grissiom/use-rtt-root-header

module: use the header in RTT_ROOT if possible