gunzip.c: Only include gzwrite on CONFIG_CMD_UNZIP

Only when we have CONFIG_CMD_UNZIP enabled do we have the 'gzwrite'
command.  While this command should be separated from CONFIG_CMD_UNZIP
we should also only include the write portion of the gz code in that
case as well.

Signed-off-by: Tom Rini <trini@konsulko.com>