Fixes:
mp/util.h:207:20: error: no template named 'function' in namespace 'std'; did you mean 'kj::Function'?
207 | using FdToArgsFn = std::function<std::vector<std::string>(int fd)>;
| ^~~~~~~~~~~~~
| kj::Function
when compiling with libc++ & _LIBCPP_REMOVE_TRANSITIVE_INCLUDES.