Mark json_fwd.hpp as exported by json.hpp for IWYU / include-cleaner

json_fwd.hpp is the only header declaring the nlohmann::json alias, so
include-what-you-use and clang-tidy's misc-include-cleaner attributed
the name to it and told users of <nlohmann/json.hpp> to include
json_fwd.hpp as well (or instead, which breaks compilation).

Fixes #5812

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -27776,7 +27776,7 @@ NLOHMANN_JSON_NAMESPACE_END
 // #include <nlohmann/detail/value_t.hpp>
 
 // #include <nlohmann/json_fwd.hpp>
-
+// IWYU pragma: export
 // #include <nlohmann/ordered_map.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++