Add missed <atomic> include in ProxyListConfiguration

[matwey]

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Changelog category (leave one):

• Build/Testing/Packaging Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Missed <atomic> is required for std::atomic<>.

[alexey-milovidov]

Split build:

Executing cmd: 'docker run --network=host --volume=/place/sandbox-data/tasks/4/5/716107354/clang-10_relwithdebuginfo_none_bundled_splitted_disable_False_binary:/package_folder --volume=/place/sandbox-data/tasks/5/0/716112705/server_log:/var/log/clickhouse-server yandex/clickhouse-split-build-smoke-test:latest >/place/sandbox-data/tasks/5/0/716112705/test_output/result.log'
Unable to find image 'yandex/clickhouse-split-build-smoke-test:latest' locally
docker: Error response from daemon: Get https://registry-1.docker.io/v2/: net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers).
See 'docker run --help'.

[alexey-milovidov]

Network issue when connecting to Docker registry.