Allow deprecated functions during DB upgrades

Fixes #1049.

I'm not sure if this is the best way to do this, but unless we want to go back through the DB upgrade routines and fix them I don't really see another way.

I also worry about someone trying to defining the WPSC_LOAD_DEPRECATED constant somewhere else, after this one has been defined, causing an error, but that may be their problem that they didn't check first.