add v11/v12 fork height

src/cryptonote_core/blockchain.cpp

@@ -91,6 +91,8 @@ static const struct {
  { 8, 6969, 0, 1524214739 },
  { 9, 53666, 0, 1538689773 },
  { 10, 63469, 0, 1541700352 },
+ { 11, 80969, 0, 1549238400 },
+ { 12, 81269, 0, 1549318761 },
 };
 
 static const struct {