Fix compiler warnings uncovered with -Wall


git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@4752 3789f03b-4d11-0410-bbf8-ca57d06f2519
diff --git a/common/rfb/ImageGetter.h b/common/rfb/ImageGetter.h
index 290249f..2e7bf37 100644
--- a/common/rfb/ImageGetter.h
+++ b/common/rfb/ImageGetter.h
@@ -25,6 +25,7 @@
   public:
     virtual void getImage(void* imageBuf,
                           const Rect& r, int stride=0) = 0;
+    virtual ~ImageGetter() {}
   };
 }
 #endif