fix build warning in bar.cpp

[zjeffer]

[1/2] Compiling C++ object waybar.p/src_bar.cpp.o
../src/bar.cpp: In function ‘Glib::ustring waybar::to_string(Gtk::PositionType)’:
../src/bar.cpp:120:1: warning: control reaches end of non-void function [-Wreturn-type]
  120 | }
      | ^