package/stress-ng: add libbsd optional dependency

stress-ng optionally detects and uses BSD wcsl* wchar string functions.

Cc: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>