Forward-declare ub_ctx/ub_result

Avoids needing unbound.h in the search path to include the
unbound_resolver.hpp header.

llarp/dns/unbound_resolver.cpp

@@ -5,6 +5,8 @@
 #include <sstream>
 #include <llarp/util/str.hpp>
 
+#include <unbound.h>
+
 namespace llarp::dns
 {
   struct PendingUnboundLookup

llarp/dns/unbound_resolver.hpp

@@ -1,6 +1,5 @@
 #pragma once
 
-#include <unbound.h>
 #include <mutex>
 #include <atomic>
 #include <memory>
@@ -17,6 +16,12 @@
 #include <uvw.hpp>
 #endif
 
+extern "C"
+{
+  struct ub_ctx;
+  struct ub_result;
+}
+
 namespace llarp::dns
 {
   using ReplyFunction =