Fix wxVector3f ctor in the propgrid sample.

Even if this code is not used, it was still wrong as it didn't initialize the
(shadowed) member variables, so fix it to actually do it.

See #16342.

samples/propgrid/propgrid.h

@@ -51,8 +51,8 @@ public:
         x = y = z = 0.0;
     }
     wxVector3f( double x, double y, double z )
+        : x(x), y(y), z(z)
     {
-        x = x; y = y; z = z;
     }
 
     double x, y, z;