dmabuf-allocator: Add missing padding in the class

This class was made subclassable, though for future growth of the code,
it's better if we have some room for add class members. Using the small
padding since this is unlikely.

gst-libs/gst/allocators/gstdmabuf.h

@@ -81,11 +81,17 @@ typedef struct _GstDmaBufAllocatorClass GstDmaBufAllocatorClass;
 struct _GstDmaBufAllocator
 {
   GstFdAllocator parent;
+
+  /*< private >*/
+  gpointer _gst_reserved[GST_PADDING];
 };
 
 struct _GstDmaBufAllocatorClass
 {
   GstFdAllocatorClass parent_class;
+
+  /*< private >*/
+  gpointer _gst_reserved[GST_PADDING];
 };