unified: fix remaining issues for building without SYS_CLOCK_EXIST

Do not include timeout_q.h when !SYS_CLOCK_EXIST, this allows removing
_unpend_thread_timing_out() in that case.

Have _abort_thread_timeout() return 0 (success) when !SYS_CLOCK_EXIST.

With this change, the minimal footprint nanokernel project compiles for
the unified kernel.

Change-Id: Ifbf9167a82fb3ebcf6941bf3f85c105c23c9060c
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
2 files changed