control bytes field in absl::flat_hash_map do not aligned to 16 bytes

in absl/container/internal/raw_hash_set.h
` using Layout = absl::container_internal::Layout;`
Layout do not align ctrl_t field to 16 bytes like this
` using Layout = absl::container_internal::Layout, slot_type>;`

it is generally fine with default allocator, which aligned to 16 bytes for a large memory region by default. but when using custom allocator (maybe some kind of memory pool), ctrl_t maybe not aligned correctly for simd to get the best performence