Zstd used an array of length 1 to mean a flexible array for C89
compatibility. Switch to a C99 flexible array to fix the UBSAN warning.

Tested locally by booting the kernel and writing to and reading from a
BtrFS filesystem with zstd compression enabled. I was unable to reproduce
the issue before the fix, however it is a trivial change.

Link: https://lkml.kernel.org/r/20231012213428.1390905-1-nickrterrell@gmail.com
Reported-by: syzbot+1f2eb3e8cd123ffce499@syzkaller.appspotmail.com
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Nick Terrell <terrelln@fb.com>
Reviewed-by: Kees Cook <keescook@chromium.org>