powerpc/bootwrapper: add fixed-head.o to simpleimage wrappers

fixed-head.o must be linked into the bootwrapper for raw-binary images to
work.  This patch adds it into the bootwrapper.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Reported-by: Eddie Dawydiuk <eddie@embeddedarm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>