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/rpc/net.cpp

@@ -7,17 +7,18 @@
 #include <chainparams.h>
 #include <clientversion.h>
 #include <core_io.h>
-#include <validation.h>
 #include <net.h>
 #include <net_processing.h>
 #include <netbase.h>
 #include <policy/policy.h>
 #include <rpc/protocol.h>
+#include <rpc/util.h>
 #include <sync.h>
 #include <timedata.h>
 #include <ui_interface.h>
-#include <util/system.h>
 #include <util/strencodings.h>
+#include <util/system.h>
+#include <validation.h>
 #include <version.h>
 #include <warnings.h>