Add dependency to mkbootimg from add_img_to_target_files

add_img_to_target_files calls mkbootimg, but required dependency is not
described in the build system. Add one more requirement to
add_img_to_target_files default so mkbootimg host tool can be always
installed when add_img_to_target_files is required.

Bug: 412477924
Test: N/A
Change-Id: I5ca6833c304fd09a65aa899c5541fcb22c828854