package/intel-gmmlib: fix license file and add hash

The license file is not named COPYING, but LICENSE.md. While we're at
it, we add the hash of the license file.

Fixes:

  http://autobuild.buildroot.net/results/09e4c14effe58ec2bc6f3deede7cc17ae6590767/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>