rpc: Include rpc/util.h where needed for RPCHelpMan

Just a preparatory commit to add the header to the includes and run
clang-format to sort the include lists.

Splitting this up into a separate commit makes future scripted-diffs
easier.

src/wallet/rpcdump.cpp

@@ -3,18 +3,19 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <chain.h>
+#include <core_io.h>
 #include <interfaces/chain.h>
 #include <key_io.h>
+#include <merkleblock.h>
 #include <rpc/server.h>
-#include <validation.h>
+#include <rpc/util.h>
 #include <script/script.h>
 #include <script/standard.h>
 #include <sync.h>
 #include <util/system.h>
 #include <util/time.h>
+#include <validation.h>
 #include <wallet/wallet.h>
-#include <merkleblock.h>
-#include <core_io.h>
 
 #include <wallet/rpcwallet.h>