LZ4: compression/decompression signedness mismatch (v2) [yank555-lu]

LZ4 compression and decompression functions require different in signedness input/output parameters:
unsigned char for compression and signed char for decompression.

Change decompression API to require "(const) unsigned char *".
v2: minor coding style fix.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

Signed-off-by: Paul Reioux <reioux@gmail.com>