Correct mistakes in #4377 and #4796

1) In c2c02f3fa99385f5a5be722fda6f71522c93bdaa, UpdateTime() should have moved to miner.cpp directly instead of pow.o.
2) After #4796, pow.cpp doesn't depend on main.h but on chain.h.
3) In b343c1a1e34f851e70649ad1f49855a7d878f9ef, there was no need to conserve 	CBlockIndex::GetBlockWork() as facade.

I'm sorry and I want to fix it.