stall_detector: fix unused function warning/error

clang unused function detection seems bugged as it complains about
unused functions in an anonymous namespace in a header: pretty weird
since we don't expect everyone who includes a header file to include
_every_ function in it, do we?

Work around it with `[[maybe_unused]]`.

tests/unit/stall_detector_test_utilities.hh

@@ -91,6 +91,7 @@ void spin_some_cooperatively(std::chrono::duration<double> how_much, void_fn bod
     }
 }
 
+[[maybe_unused]]
 future<> spin_some_cooperatively_coro(std::chrono::duration<double> how_much, void_fn body = []{}) {
     auto end = std::chrono::steady_clock::now() + how_much;
     while (std::chrono::steady_clock::now() < end) {