Make mixin optional with default for rpc transfer

src/wallet/wallet_rpc_server_commands_defs.h

@@ -398,7 +398,7 @@ namespace wallet_rpc
         KV_SERIALIZE(account_index)
         KV_SERIALIZE(subaddr_indices)
         KV_SERIALIZE(priority)
-        KV_SERIALIZE(mixin)
+        KV_SERIALIZE_OPT(mixin, (uint64_t)0)
         KV_SERIALIZE_OPT(ring_size, (uint64_t)0)
         KV_SERIALIZE(unlock_time)
         KV_SERIALIZE(payment_id)
@@ -455,7 +455,7 @@ namespace wallet_rpc
         KV_SERIALIZE(account_index)
         KV_SERIALIZE(subaddr_indices)
         KV_SERIALIZE(priority)
-        KV_SERIALIZE(mixin)
+        KV_SERIALIZE_OPT(mixin, (uint64_t)0)
         KV_SERIALIZE_OPT(ring_size, (uint64_t)0)
         KV_SERIALIZE(unlock_time)
         KV_SERIALIZE(payment_id)
@@ -568,7 +568,7 @@ namespace wallet_rpc
         KV_SERIALIZE(account_index)
         KV_SERIALIZE(subaddr_indices)
         KV_SERIALIZE(priority)
-        KV_SERIALIZE(mixin)
+        KV_SERIALIZE_OPT(mixin, (uint64_t)0)
         KV_SERIALIZE_OPT(ring_size, (uint64_t)0)
         KV_SERIALIZE(unlock_time)
         KV_SERIALIZE(payment_id)
@@ -630,7 +630,7 @@ namespace wallet_rpc
       BEGIN_KV_SERIALIZE_MAP()
         KV_SERIALIZE(address)
         KV_SERIALIZE(priority)
-        KV_SERIALIZE(mixin)
+        KV_SERIALIZE_OPT(mixin, (uint64_t)0)
         KV_SERIALIZE_OPT(ring_size, (uint64_t)0)
         KV_SERIALIZE(unlock_time)
         KV_SERIALIZE(payment_id)