iwyu fix https://github.com/bitcoin/bitcoin/actions/runs/21266012856/job/61207857203?pr=25722

Author: ryanofsky

src/kernel/bitcoinkernel.cpp

@@ -46,8 +46,8 @@
 #include <span>
 #include <stdexcept>
 #include <string>
-#include <tuple>
 #include <utility>
+#include <variant>
 #include <vector>
 
 using kernel::ChainstateRole;