Add missing break statement to switch-case

This is the last and only build-time warning from compiler:

[11/23] Building CXX object src/CMakeFiles/kcm_sddm.dir/themesmodel.cpp.o
/kde/src/plasma/sddm-kcm/src/themesmodel.cpp: In member function ‘virtual QVariant ThemesModel::data(const QModelIndex&, int) const’:
/kde/src/plasma/sddm-kcm/src/themesmodel.cpp:96:9: warning: this statement may fall through [-Wimplicit-fallthrough=]
   96 |         if (metadata.supportsBackground()) {
      |         ^~
/kde/src/plasma/sddm-kcm/src/themesmodel.cpp:99:5: note: here
   99 |     case DeletableRole:
      |     ^~~~