refactor: move m_initial_sync_finished out of header

The PeerManager implementation was moved into the source file in
bitcoin#20811 (dash#5352) but the PR that introduced `m_initial_sync_finished`
bitcoin#19858 (dash#5869) was merged later and didn't account for the
out-of-order backport. We need to correct for that manually.

Author: kwvg

src/net_processing.cpp

@@ -524,6 +524,10 @@ class PeerManagerImpl final : public PeerManager
     /** Whether this node is running in blocks only mode */
     const bool m_ignore_incoming_txs;
 
+    /** Whether we've completed initial sync yet, for determining when to turn
+      * on extra block-relay-only peers. */
+    bool m_initial_sync_finished{false};
+
     /** Protects m_peer_map. This mutex must not be locked while holding a lock
      *  on any of the mutexes inside a Peer object. */
     mutable Mutex m_peer_mutex;

src/net_processing.h

@@ -116,11 +116,6 @@ class PeerManager : public CValidationInterface, public NetEventsInterface
                                 const std::chrono::microseconds time_received, const std::atomic<bool>& interruptMsgProc) = 0;
 
     virtual bool IsBanned(NodeId pnode) = 0;
-
-    /** Whether we've completed initial sync yet, for determining when to turn
-      * on extra block-relay-only peers. */
-    bool m_initial_sync_finished{false};
-
 };
 
 #endif // BITCOIN_NET_PROCESSING_H