lz4: fix CC

It defined the CC to ${TARGET_PREFIX}gcc which lost the --sysroot and was
incorrect, it would cause unexpected errors, we should define it as CC=${CC}.

[YOCTO #5869]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>