Replace CBlockHeader::GetHash with call to SerializeHash
Replace CBlockHeader::GetHash with call to SerializeHash

Removes variability between LE and BE.
As suggested by @sipa.

adapted from bitcoin@a0ae79d77552d9ee67469c934458a3f3bd170858
PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,Darknet-Crypto/Darknet,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX,Darknet-Crypto/Darknet,PIVX-Project/PIVX,PIVX-Project/PIVX