Silence Clang 18 warning

[zanderso]

The Clang 18 roll here flutter/engine#45486 will add a new warning like:

we get errors of the form:

../../third_party/json/include\nlohmann/json.hpp(5142,35): error: identifier '_json' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
 5142 | inline nlohmann::json operator "" _json(const char* s, std::size_t n)
      |                       ~~~~~~~~~~~~^~~~~
      |                       operator""_json

That is, Clang rejects operator "" _a unless spaces are removed. This PR adds a flag to silence the warning util it is fixed upstream.

[jonahwilliams]

LGTM