pulsecore/shm: Remove shm_marker struct packing for pa_atomic_t fields

Taking addresses of fields in a packed struct are not guaranteed to be
aligned, resulting in warnings such as:

    ../src/pulsecore/shm.c: In function 'sharedmem_create':
    ../src/pulsecore/shm.c:198:25: error: taking address of packed member of 'struct shm_marker' may result in an unaligned pointer value [-Werror=address-of-packed-member]
      198 |         pa_atomic_store(&marker->pid, (int) getpid());
          |                         ^~~~~~~~~~~~

The struct already has its fields and types laid out in such a way that
the desired packing (without padding) is guaranteed - enforce this with
a `static_assert` to get rid of the unaligned pointer warning.
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