Fix compilation for AVX512

[burgerindividual]

When compiling for AVX512, the following error occurs in bitmask.rs:

error[E0599]: no associated item named `EMPTY_BIT_MASK` found for associated type `<lane_count::LaneCount<N> as lane_count::SupportedLaneCount>::BitMask` in the current scope
   --> /home/ryan/.cargo/git/checkouts/portable-simd-8edd7e7a3748e100/3439347/crates/core_simd/src/masks/bitmask.rs:133:72
    |
133 |         let mut bytes = <LaneCount<N> as SupportedLaneCount>::BitMask::EMPTY_BIT_MASK;
    |                                                                        ^^^^^^^^^^^^^^ associated item not found in `<lane_count::LaneCount<N> as lane_count::SupportedLaneCount>::BitMask`

For more information about this error, try `rustc --explain E0599`.

This just fixes the reference to EMPTY_BIT_MASK to match everywhere else.

[calebzulawski]

Thank you!