When implementing assignment, also implement copy ctor

Summary:
/plasma-desktop/kcms/keyboard/xkb_rules.cpp: In static member function ‘static Rules::GeometryId Rules::getGeometryId(const QString&)’:
/plasma-desktop/kcms/keyboard/xkb_rules.cpp:426:16: warning: implicitly-declared ‘Rules::GeometryId::GeometryId(const Rules::GeometryId&)’ is deprecated [-Wdeprecated-copy]
  426 |         return defaultGeoId;
      |                ^~~~~~~~~~~~

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23042