Fix for jsoncpp deprecated function warning

When compiling with a newer version of jsoncpp (and
ENABLE_SYSTEM_JSONCPP=true), jsoncpp emits a warning
about a deprecated function that minetest uses.

src/script/common/c_content.cpp

@@ -1250,8 +1250,13 @@ static bool push_json_value_helper(lua_State *L, const Json::Value &value,
  lua_newtable(L);
  for (Json::Value::const_iterator it = value.begin();
  it != value.end(); ++it) {
+#ifndef JSONCPP_STRING
  const char *str = it.memberName();
  lua_pushstring(L, str ? str : "");
+#else
+ std::string str = it.name();
+ lua_pushstring(L, str.c_str());
+#endif
  push_json_value_helper(L, *it, nullindex);
  lua_rawset(L, -3);
  }