Add virtual destructor to IBackgroundBlockEmerger to silence warning

src/emerge.h

@@ -75,6 +75,7 @@ class IBackgroundBlockEmerger
 public:
 	virtual bool enqueueBlockEmerge(u16 peer_id, v3s16 p,
 			bool allow_generate) = 0;
+	virtual ~IBackgroundBlockEmerger() {}
 };
 
 class EmergeManager : public IBackgroundBlockEmerger {