std::unary_function is deprecated

This warning appears starting with gcc 12.

../../../../src/lib/asiolink/testutils/timed_signal.h:54:43: warning: ‘template<class _Arg, class _Result> struct std::unary_function’ is deprecated [-Wdeprecated-declarations]
54 |     class SendSignalCallback: public std::unary_function<void, void> {
|                                           ^~~~~~~~~~~~~~
In file included from /usr/include/c++/12.1.0/bits/unique_ptr.h:37,
from /usr/include/c++/12.1.0/memory:76,
from /usr/include/boost/smart_ptr/detail/sp_counted_impl.hpp:35,
from /usr/include/boost/smart_ptr/detail/shared_count.hpp:27,
from /usr/include/boost/smart_ptr/shared_ptr.hpp:17,
from /usr/include/boost/shared_ptr.hpp:17,
from ../../../../src/lib/asiolink/interval_timer.h:10,
from ../../../../src/lib/asiolink/testutils/timed_signal.h:12:
/usr/include/c++/12.1.0/bits/stl_function.h:117:12: note: declared here
117 |     struct unary_function
|            ^~~~~~~~~~~~~~

According to cppreference dot com, the function is deprecated in C++11 and removed in C++17.