build+src: mark destructor virtual in classes with virtual functions

And to prevent similar mistakes in the future, add -Wnon-virtual-dtor
to the default build flags.

Change-Id: I28e2361341abab29fca134309288cd259736d67a
diff --git a/src/security/v2/trust-anchor-group.cpp b/src/security/v2/trust-anchor-group.cpp
index 6b94951..24f5b9e 100644
--- a/src/security/v2/trust-anchor-group.cpp
+++ b/src/security/v2/trust-anchor-group.cpp
@@ -24,7 +24,6 @@
 #include "util/io.hpp"
 #include "util/logger.hpp"
 
-#include <set>
 #include <boost/filesystem.hpp>
 #include <boost/range/adaptor/map.hpp>
 #include <boost/range/algorithm/copy.hpp>
@@ -44,6 +43,8 @@
 {
 }
 
+TrustAnchorGroup::~TrustAnchorGroup() = default;
+
 size_t
 TrustAnchorGroup::size() const
 {