Add a couple of missing default virtual destructors

include/loot/database_interface.h

@@ -40,6 +40,8 @@ namespace loot {
 /** @brief The interface provided by API's database handle. */
 class DatabaseInterface {
 public:
+  virtual ~DatabaseInterface() = default;
+
   /**
    * @name Data Reading & Writing
    * @{

include/loot/plugin_interface.h

@@ -39,6 +39,8 @@ namespace loot {
  */
 class PluginInterface {
 public:
+  virtual ~PluginInterface() = default;
+
   /**
    * Get the plugin's filename.
    * @return The plugin filename.