Merge #113: Add missing include to util.h

6929c40 Add missing include to util.h (fanquake)

Pull request description:

  Fixes:
  ```bash
  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`.

ACKs for top commit:
  ryanofsky:
    Code review ACK 6929c40, thanks for the fix
  hebasto:
    ACK 6929c40, tested on Ubuntu 23.10 by building Bitcoin Core with depends using the following patch:

Tree-SHA512: 9ad6097bd1f3e0f7ee213e5c5bd17be56fa06c23ff25bbcb0122c40ecff71411417826946fd9c4c1d7ef302d7fd771a79fb81f2d240fc22e47d8ac2f40cce85e

Author: ryanofsky

include/mp/util.h

@@ -7,6 +7,7 @@
 
 #include <capnp/schema.h>
 #include <cstddef>
+#include <functional>
 #include <future>
 #include <kj/common.h>
 #include <kj/exception.h>