include/kernel: check type of object passed to K_POLL_EVENT_INITIALIZER

This prevents accidentally passing the wrong pointer type (e.g. one level
of indirection too many) as the object `void *`.

Co-authored-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>