Version 3.11 of nlohmann/json introduced "versioned, ABI-tagged inline
namespace"s, which breaks our forward declaration. Fortunately, we can
assume the json_fwd.hpp header to be present starting from that same
version because the JSON_MultipleHeaders option now defaults to ON and
even if not, json_fwd.hpp is installed since patch version 3.11.2. For
earlier versions, both methods work but json_fwd.hpp isn't guaranteed
to be installed. Still use it if available.

Fixes root-project#11130

(cherry picked from commit ed56a35)

Our forward declaration in REveElement.hxx breaks with the versioned
namespaces in 3.11, so we require the json_fwd.hpp header.

(cherry picked from commit ee52a8b)