third_party: bump libmemcached to the latest version
third_party: bump libmemcached to the latest version

The main reason to bump a version is a fix of build on Ubuntu 20.04,
see [1].

1. https://github.com/tarantool/libmemcached/commit/dc5b8fa6e5a11ca5800cafb60a23ff908f23f025

Fixes #80
Fixes #56

tarantool/memcached,tarantool/memcached