The relative directory paths in EXTRA_CFLAGS will make compile look for the files in kernel-headers directory tree. The files are found only in full kernel-source tree. Edit the EXTRA_CFLAGS to contain absolute path to full kernel-source tree

EXTRA_CFLAGS += -I/usr/src/kernel-power-source/drivers/...

Again, I suspect there is a better way to accomplish this without Makefile editing, but this should get you forward.