data: Fix schema generation for strict checks

org.gnome.Disks.gschema.xml:7:31  Error on line 7 char 31: Only one <summary> element allowed inside <key>.  --strict was specified; exiting.
org.gnome.settings-daemon.plugins.gdu-sd.gschema.xml:7:31  Error on line 7 char 31: Only one <summary> element allowed inside <key>.  --strict was specified; exiting.

https://bugzilla.gnome.org/show_bug.cgi?id=772863