x86, boot: correct the calculation of ZO_INIT_SIZE

Correct the calculation of ZO_INIT_SIZE (the amount of memory we need
during decompression).  One symbol (ZO_startup_32) was missing from
zoffset.h, and another (ZO_z_extract_offset) was misspelled.

[ Impact: build fix ]

Signed-off-by: H. Peter Anvin <hpa@zytor.com>