package/googlefontdirectory: silence false positive check-package warning

check-package OverriddenVariable check believes we are overriding the
value of GOOGLEFONTDIRECTORY_LICENSE, but in fact we are not. Let's
tell check-package not to complain about this.

Fixes:

package/googlefontdirectory/googlefontdirectory.mk:28: unconditional override of variable GOOGLEFONTDIRECTORY_LICENSE previously conditionally set

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