account: fix build after removal of rest/oauth-proxy.h

librest has removed its OAuth 1 support. This is OK, because we do not use it. We're just including the wrong header.

~~Note: I am going to violate our usual policies here and just merge this without waiting for Will, because this is a one-line change and I'm reasonably confident he will probably not mind. ~~ And we have a lot broken right now: it will take days to sort out if I wait for everyone....