message: Fix incompatible pointer type on arm
  On x86 platform char type is `i8` while on arm it is `u8`, change
  to use c_char to avoid compilation issue.
diff: use std::io::Result;
  +use libc::c_char;
  use lz4_sys::{LZ4_compressBound, LZ4_compress_default, LZ4_decompress_safe};
  use nydus_utils::einval;
  @@ -18,8 +19,8 @@ pub(super) fn lz4_compress(src: &[u8]) -> Result<Vec<u8>> {
  let mut dst_buf = Vec::with_capacity(compress_bound as usize);
  let dec_size = unsafe {
  LZ4_compress_default(
  - src.as_ptr() as *const i8,
  - dst_buf.as_mut_ptr() as *mut i8,
  + src.as_ptr() as *const c_char,
  + dst_buf.as_mut_ptr() as *mut c_char,
  src.len() as i32,
  compress_bound,
  )
  @@ -46,8 +47,8 @@ pub(super) fn lz4_decompress(src: &[u8], dst: &mut [u8]) -> Result<usize> {
  let dec_bytes = unsafe {
  LZ4_decompress_safe(
  - src.as_ptr() as *const i8,
  - dst.as_mut_ptr() as *mut i8,
  + src.as_ptr() as *const c_char,
  + dst.as_mut_ptr() as *mut c_char,
  src.len() as i32,
  size,
  )
  