[POWERPC] boot: Record header bytes in gunzip_start

Record the number of header bytes skipped in the total bytes read field.

This is needed for the initramfs parsing code to find the end of the zip file.

Signed-off-by: Milton Miller <miltonm@bga.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>