build: Make sure sysgen related header files get updated

Since sysgen copies kernel/microkernel/include/micro_private_types.h and
kernel/microkernel/include/kernel_main.h we need to make sure that
sysgen gets invoked if those files ever change.  Otherwise we might have
stale versions.

Change-Id: Id84522e3af693f1323f73c9642d1884f0be4b7cf
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>