Add explicit virtual dtor
Add explicit virtual dtor

Add explicit virtual dtor to GraphBuilderInterface. This avoids tripping
a -Wnon-virtual-dtor warning, and also ensures that the correct
(virtual) dtor is called if an instance is deleted via a pointer to the
base type.

loot/yaml-cpp,loot/yaml-cpp,loot/yaml-cpp,dand-oss/yaml-cpp,jbeder/yaml-cpp,jbeder/yaml-cpp,dand-oss/yaml-cpp,WrinklyNinja/yaml-cpp,tangrams/yaml-cpp,SimplyKnownAsG/yaml-cpp,tangrams/yaml-cpp,jbeder/yaml-cpp,WrinklyNinja/yaml-cpp,tangrams/yaml-cpp,dand-oss/yaml-cpp,WrinklyNinja/yaml-cpp,loot/yaml-cpp,graehl/yaml-cpp,SimplyKnownAsG/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,graehl/yaml-cpp,tangrams/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,graehl/yaml-cpp,SimplyKnownAsG/yaml-cpp,dand-oss/yaml-cpp,SimplyKnownAsG/yaml-cpp,jbeder/yaml-cpp,WrinklyNinja/yaml-cpp