socat: bump to version 2.0.0-b6

2.0.0-b6 fix a security issue:
Under certain circumstances an FD leak occurs and may be misused for denial of
service attacks against socat running in server mode.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>