sysstat: add UPSTREAM_VERSION_UNKNOWN

sysstat: add UPSTREAM_VERSION_UNKNOWN

We are taking a development version from an odd-even versioned project,
so upstream version check reports that the latest version is less than
the one the recipe is using.

UPSTREAM_VERSION_UNKNOWN avoids a version check failure until
there is a new stable series.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>