samba4: add optional libbsd dependency

It's used for some small functions like md5 support, non-essential since
samba has an internal fallback for those, but still add it for
predictability.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>