selftests/damon: add damon to selftests root Makefile

Currently the damon selftests are not built with the rest of the
selftests. We add damon to the list of targets.

Fixes: b348eb7a ("mm/damon: add user space selftests")
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Yuanchu Xie <yuanchu@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>