http: add missing header bootlegged by boost < 1.72

Summary:

httpserver.cpp:74:10: error: no template named 'deque' in namespace
'std'
    std::deque<std::unique_ptr<WorkItem>> queue;
    ~~~~~^

Backport of core PR17249.

Fixes the build on ArchLinux.

Test Plan:

ninja check
make check

Reviewers: #bitcoin_abc, markblundeberg

Reviewed By: #bitcoin_abc, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D5161