thread-safety: add missing lock annotation

No warning is currently emitted because our reverse_lock does not enforce our
thread-safety annotations. Once it is fixed, the unlock would cause a warning.

src/node/interfaces.cpp

@@ -431,7 +431,7 @@ public:
 };
 
 // NOLINTNEXTLINE(misc-no-recursion)
-bool FillBlock(const CBlockIndex* index, const FoundBlock& block, UniqueLock<RecursiveMutex>& lock, const CChain& active, const BlockManager& blockman)
+bool FillBlock(const CBlockIndex* index, const FoundBlock& block, UniqueLock<RecursiveMutex>& lock, const CChain& active, const BlockManager& blockman) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
 {
     if (!index) return false;
     if (block.m_hash) *block.m_hash = index->GetBlockHash();