mkimage: Include missing files in build dependency calculations

Include default_image.o and fit_image.o into the build dependency
calculations. This makes sure they get rebuilt if any of the headers
they include are modified

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Ron Lee <ron@debian.org>

Edited commit message.

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