[#3068] fix shadowed member

[#3068] fix shadowed member

data.cc: In member function ‘virtual void isc::data::MapElement::toJSON(std::ostream&) const’:
data.cc:919:51: warning: declaration of ‘m’ shadows a member of ‘isc::data::MapElement’ [-Wshadow]
  919 |     const std::map<std::string, ConstElementPtr>& m = mapValue();
      |                                                   ^
In file included from data.cc:9:
../../../src/lib/cc/data.h:769:44: note: shadowed declaration is here
  769 |     std::map<std::string, ConstElementPtr> m;
      |                                            ^