Fix compiler warning in mapviz widgets.h
Fix compiler warning in mapviz widgets.h

QListWidget::count() returns an int, so comparing it to a size_t causes a sign-compare warning.
jatowler/mapviz,swri-robotics/mapviz,evenator/mapviz,evenator/mapviz,jatowler/mapviz,swri-robotics/mapviz