rpc: call IsInitialBlockDownload via miner interface

src/node/interfaces.cpp

@@ -847,6 +847,11 @@ public:
         return chainman().GetParams().IsTestChain();
     }
 
+    bool isInitialBlockDownload() override
+    {
+        return chainman().IsInitialBlockDownload();
+    }
+
     std::optional<uint256> getTipHash() override
     {
         LOCK(::cs_main);