Skip WallClock.string_for_datetime in gnome-desktop-3.0.vapi

This is a walk around for build problem:

gnome-desktop-3.0.vapi:211.57-211.64: error: The symbol `GDesktop' could not be found

(cherry picked from commit 0ce144e7146f32bd4e52d225b5b128a25c36c532)