fix build error due to missing gweather-xml.h
fix build error due to missing gweather-xml.h

GWeather doesn't export the deprecated GWeatherXML anymore since
commit 354f1aa9003ee08c6e63ecbaaa6b9179b9178184 . Luckily, our last
usage of it is long gone. Remove leftover include.

See https://bugzilla.redhat.com/show_bug.cgi?id=890219

GNOME/gnome-panel,GNOME/gnome-panel