Fix -Wunused warnings on JSON_DIAGNOSTICS (#2976)

* Fix #2975

Define JSON_DIAGNOSTICS to 0 if not defined to fix annoying Wundef warnings.

* amalgamated

single_include/nlohmann/json.hpp

@@ -2514,6 +2514,10 @@ JSON_HEDLEY_DIAGNOSTIC_POP
     #define JSON_EXPLICIT explicit
 #endif
 
+#ifndef JSON_DIAGNOSTICS
+    #define JSON_DIAGNOSTICS 0
+#endif
+
 
 namespace nlohmann
 {