lib/power: fix make build error

In a fix backported from 21.02, there was no modification to
the Makefile included, as that file does not exist any more.
There is a change needed to Makefile in 19.11 as that still
uses make.

Fixes: b00e2adc ("power: export guest channel header file")

Signed-off-by: David Hunt <david.hunt@intel.com>