Rearrange proof/service node forward declarations to help gcc-8

gcc-8's unique_ptr apparently isn't okay with a forward declared type.
This reverses how proof and service_node_list include/fwd declare each
other to get around it.

src/cryptonote_core/service_node_list.h

@@ -38,6 +38,7 @@
 #include "cryptonote_core/service_node_voting.h"
 #include "cryptonote_core/service_node_quorum_cop.h"
 #include "common/util.h"
+#include "uptime_proof.h"
 
 namespace cryptonote
 {
@@ -46,11 +47,6 @@ class BlockchainDB;
 struct checkpoint_t;
 }; // namespace cryptonote
 
-namespace uptime_proof
-{
-  class Proof;
-}
-
 namespace service_nodes
 {
   constexpr uint64_t INVALID_HEIGHT = static_cast<uint64_t>(-1);

src/cryptonote_core/uptime_proof.cpp

@@ -1,4 +1,5 @@
 #include "uptime_proof.h"
+#include "service_node_list.h"
 #include "common/string_util.h"
 #include "version.h"
 

src/cryptonote_core/uptime_proof.h

@@ -1,9 +1,14 @@
 #pragma once
 
-#include "service_node_list.h"
-#include "../cryptonote_protocol/cryptonote_protocol_defs.h"
+#include "cryptonote_protocol/cryptonote_protocol_defs.h"
 #include <oxenmq/bt_serialize.h>
 
+namespace service_nodes {
+
+struct service_node_keys;
+
+}
+
 namespace uptime_proof
 {