xprops: free the text list content too

A text list is an array of pointers into a buffer. Freeing the
array is not enough, we need to free the buffer too.

https://bugzilla.gnome.org/show_bug.cgi?id=729732

src/x11/xprops.c

@@ -753,7 +753,7 @@ text_property_to_utf8 (Display *xdisplay,
   ret = g_strdup (local_list[0]);
 
  out:
-  meta_XFree (local_list);
+  XFreeStringList (local_list);
   return ret;
 }