ip_resolver_t: Silencing C++-warning -Wnon-virtual-dtor (#3837)

* ip_resolver_t: adding virtual descructor to silence C++-warning -Wnon-virtual-dtor

* adding my relicense

RELICENSE/Ricardicus.md

+Permission to Relicense under MPLv2 or any other OSI approved license chosen by the current ZeroMQ BDFL
+This is a statement by Rickard Hallerbäck that grants permission to relicense its copyrights in the libzmq C++ library (ZeroMQ) under the Mozilla Public License v2 (MPLv2) or any other Open Source Initiative approved license chosen by the current ZeroMQ BDFL (Benevolent Dictator for Life).
+
+A portion of the commits made by the Github handle "Ricardicus", with commit author "Rickard Hallerbäck", are copyright of Rickard Hallerbäck. This document hereby grants the libzmq project team to relicense libzmq, including all past, present and future contributions of the author listed above.
+
+Rickard Hallerbäck 2020/02/28
\ No newline at end of file

src/ip_resolver.hpp

@@ -90,6 +90,7 @@ class ip_resolver_t
 {
   public:
     ip_resolver_t (ip_resolver_options_t opts_);
+    virtual ~ip_resolver_t (){};
 
     int resolve (ip_addr_t *ip_addr_, const char *name_);