.gitignore: Add ZSTD-compressed files

For now, that's arch/x86/boot/compressed/vmlinux.bin.zst but probably more
will come, thus let's be consistent with all other compressors.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Nick Terrell <terrelln@fb.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200730190841.2071656-8-nickrterrell@gmail.com