package/c-ares: use LICENSE.md

c-ares has a LICENSE.md file since version 1.12 and
https://github.com/c-ares/c-ares/commit/4e861351d9deaef7b78aee50ce9229325f4fc59a



So use it instead of one of the source file and add its hash

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>