ARM: 8970/1: decompressor: increase tag size

The size field of the tag header structure is supposed to be set to the
size of a tag structure including the header.

Fixes: c7725687 ("ARM: add additional table to compressed kernel")
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>