Fix slog warning

[lo-simon]

Use char* for slog::log to prevent warning.

[garethsb]

Were you able to work out why this overload doesn't work in this case?

nmos-cpp/Development/nmos/slog.h

Line 15 in f549712

inline log_statement& operator<<(log_statement& s, const utf16string& u16s)

[lo-simon]

Were you able to work out why this overload doesn't work in this case?

nmos-cpp/Development/nmos/slog.h

Line 15 in f549712

inline log_statement& operator<<(log_statement& s, const utf16string& u16s)

The overload does work, but I keep getting the following warning,
warning C4239: nonstandard extension used: argument: conversion from slog::detail::anonymous-namespace::log<slog::base_gate,20> to slog::log_statement & that why I made the change.