Rename gsettings-desktop-schemas VAPI file

Vala recommends using the same name used in the pkg-config .pc file
for the VAPI file. This implies that the gsettings-desktop-schemas
VAPI file should be called gsettings-desktop-schemas.vapi instead of
gdesktop-enums-3.0.vapi.

This patch changes the file accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=789045