utils: Fix run-time icon warning

In newer versions of gtk+, gtk_icon_theme_lookup_by_gicon_for_scale()
will warn if somebody uses GTK_ICON_LOOKUP_GENERIC_FALLBACK with
functions that don't support it.

The warning looks like:
  (org.gnome.Maps:13949): Gtk-WARNING **: (gtkicontheme.c:5432):
  gtk_icon_theme_lookup_by_gicon_for_scale: runtime check failed:
  ((flags & GTK_ICON_LOOKUP_GENERIC_FALLBACK) == 0)

https://bugzilla.gnome.org/show_bug.cgi?id=760444