HPCC-12728 The memcached plugin does not compiling on Ubuntu 12.04

Because the version of libmemcached headers is too old.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
I will change it (though I tend to avoid calling variables 'error' for two reasons - one is that it's a return code which may also indicate success or other non-error statuses, the other is that there is a standard C function called error that tends to be in scope from stdlib, and which can lead to confusing messages/autocomplete hints)