image: Fixes build warning with CONFIG_FIT_IMAGE_POST_PROCESS

image: Fixes build warning with CONFIG_FIT_IMAGE_POST_PROCESS

The function 'board_fit_image_post_process' is defined only when the
config option CONFIG_FIT_IMAGE_POST_PROCESS is enabled. For secure
systems that do not use SPL but do use FIT kernel images, only
CONFIG_FIT_IMAGE_POST_PROCESS will be defined, which will result in an
implicit declaration of function 'board_fit_image_post_process' warning
while building u-boot. Fix this warning.

Signed-off-by: Madan Srinivas <madans@ti.com>
Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>