posix_timers missing from multimedia and poky-altcfg builds

posix_timers is a test program that can be built from kernel source:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/timers/posix_timers.c

It's required by the Timer test which is a part of pre-merge suite:
https://github.com/qualcomm-linux/qcom-linux-testkit/blob/main/Runner/suites/Kernel/Baseport/Timer/run.sh