package/gnuchess: license is GPL-3.0+, not GPL-2.0+

gnuchess is licensed under GPL-3.0+ not GPL-2.0+, also add hash for
license file

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

package/gnuchess/gnuchess.hash

@@ -1,2 +1,3 @@
 # sha256 locally computed
 sha256 3c425c0264f253fc5cc2ba969abe667d77703c728770bd4b23c456cbe5e082ef  gnuchess-6.2.4.tar.gz
+sha256 8ceb4b9ee5adedde47b31e975c1d90c73ad27b6b165a1dcd80c7c545eb65b903  COPYING

package/gnuchess/gnuchess.mk

@@ -6,7 +6,7 @@
 
 GNUCHESS_VERSION = 6.2.4
 GNUCHESS_SITE = $(BR2_GNU_MIRROR)/chess
-GNUCHESS_LICENSE = GPL-2.0+
+GNUCHESS_LICENSE = GPL-3.0+
 GNUCHESS_LICENSE_FILES = COPYING
 
 GNUCHESS_DEPENDENCIES = host-flex flex