cmd_ximg.c: fix error: 'i' undeclared

Fix compile error:

	cmd_ximg.c: In function 'do_imgextract':
	cmd_ximg.c:234: error: 'i' undeclared (first use in this function)
	cmd_ximg.c:234: error: (Each undeclared identifier is reported only once
	cmd_ximg.c:234: error: for each function it appears in.)

Signed-off-by: Wolfgang Denk <wd@denx.de>