Add a virtual destructor for XdgSurfaceConfigure

It's inherited by XdgToplevelConfigure and we need to make sure that the
object gets destroyed properly.
Alternatively we get a new-delete-type-mismatch error in ASAN and
rightfully so.

src/xdgshellclient.h

@@ -34,6 +34,8 @@ class AbstractOutput;
 class XdgSurfaceConfigure
 {
 public:
+    virtual ~XdgSurfaceConfigure() {}
+
     enum ConfigureField {
         PositionField = 0x1,
         SizeField = 0x2,