Add primitive implementation of GetSaplingAnchorEnd.

src/chain.cpp

@@ -7,6 +7,13 @@
 
 using namespace std;
 
+uint256 CBlockIndex::GetSaplingAnchorEnd() const {
+    // TODO: The block header's hashSaplingAnchorEnd is only guaranteed to
+    // be valid on or after the Sapling activation height.
+
+    return hashSaplingAnchorEnd;
+}
+
 /**
  * CChain implementation
  */