pidfd-util: force alignment of file_handle union to avoid assert on 32bit

pidfd-util: force alignment of file_handle union to avoid assert on 32bit

On 32bit the union won't be aligned automatically, needs to be enforced:

Assertion '((uintptr_t) _p) % alignof(uint64_t) == 0' failed at src/basic/pidfd-util.c:251, function pidfd_get_inode_id_impl(). Aborting.

Follow-up for 9c039ef5ff7f683aa9f7a36cb39bac94a12964b2