net: lib: ptp: fix memory slab alignment issues

Commit 3c47f91b introduced alignment
validation in K_MEM_SLAB_DEFINE macros.
A couple PTP message slabs wer using alignment 8, while the size of
the elements wasn't a multiple of 8, causing a static assertion
failure.

Fix by changing the alignment from 8 to 4 bytes.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>