validation: Use span for ProcessNewBlockHeaders

Makes it friendlier for potential future users of the kernel library if
they do not store the headers in a std::vector, but can guarantee
contiguous memory.

src/rpc/mining.cpp

@@ -1095,7 +1095,7 @@ static RPCHelpMan submitheader()
     }
 
     BlockValidationState state;
-    chainman.ProcessNewBlockHeaders({h}, /*min_pow_checked=*/true, state);
+    chainman.ProcessNewBlockHeaders({{h}}, /*min_pow_checked=*/true, state);
     if (state.IsValid()) return UniValue::VNULL;
     if (state.IsError()) {
         throw JSONRPCError(RPC_VERIFY_ERROR, state.ToString());