efi: libstub: Fix incorrect payload size in zboot header

The linker script symbol definition that captures the size of the
compressed payload inside the zboot decompressor (which is exposed via
the image header) refers to '.' for the end of the region, which does
not give the correct result as the expression is not placed at the end
of the payload. So use the symbol name explicitly.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>