osv-memcached: Fixes in a Makefile making the developing easier

 - Don't delete old sources every time the target is built.
 - Make the module rebuilt when the sources have changed.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
osv-memcached: remove the not needed parameter

osv-memcached is using the "shrinker" interface now to triger the memory freeing,
thus no need for any "max-memory size" semantics paramater. It's the total
amount of free Guest memory and that will control the application memory usage.

Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>