Build fails on x86_64 musl

[gmbeard]

The build failures appear to be caused by the use of strerror()...

/builddir/shadow-cast-0.6.3/src/io/unix_socket.cpp:71:63: error: 'strerror' was not declared in this scope; did you mean 'stderr'?
   71 |         throw std::runtime_error { "UnixSocket::accept() "s + strerror(errno) };
      |                                                               ^~~~~~~~
      |                                                               stderr

/builddir/shadow-cast-0.6.3/src/io/unix_socket.cpp:79:63: error: 'strerror' was not declared in this scope; did you mean 'stderr'?
   79 |         throw std::runtime_error { "UnixSocket::listen() "s + strerror(errno) };
      |                                                               ^~~~~~~~
      |                                                               stderr