[backport#15435] Merge #15435: rpc: Add missing #include

Summary:
39e20fc54f Add missing #include. (Daniel Kraft)

Pull request description:

bd0dbe8763fc3029cf96531c9ccaba280b939445 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`.

Tree-SHA512: 75c03cfadb28a309d6deb36feeb0ee6ce0b38e8a1176919bc611ea720feff8c42ec9ed0ac8ab74ba9c531a3b7ec9ccbed0c8692ebdf5f9fc17867b9750a1d9f6

Backport of Core PR15435

Test Plan:

ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6270