image: add missing DTC include-prefixes directory
include-prefixes were moved to a common directory in linux v4.12, see
https://github.com/torvalds/linux/commit/d5d332d3f7e8435e264a71b90178dee69428d630
This is needed for bcm27xx kernel v6.6 support.
Signed-off-by: Álvaro Fernández Rojas <[email protected]>