Add missing #include.

bd0dbe8763 introduced a dependency of
rpc/util.h on RPCErrorCode, defined in rpc/protocol.h.  The latter file
is only included from rpc/util.cpp, though.  This commit fixes the
missing include, by moving the #include of rpc/protocol.h to
rpc/util.h.

src/rpc/util.cpp

@@ -5,7 +5,6 @@
 #include <key_io.h>
 #include <keystore.h>
 #include <policy/fees.h>
-#include <rpc/protocol.h>
 #include <rpc/util.h>
 #include <tinyformat.h>
 #include <util/strencodings.h>

src/rpc/util.h

@@ -7,6 +7,7 @@
 
 #include <node/transaction.h>
 #include <pubkey.h>
+#include <rpc/protocol.h>
 #include <script/standard.h>
 #include <univalue.h>