gmp: correct legal info

Since version 6, GMP is distributed under the dual licenses LGPLv3+ or
GPLv2+

Reviewed-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/gmp/gmp.mk

@@ -8,8 +8,8 @@ GMP_VERSION = 6.1.1
 GMP_SITE = $(BR2_GNU_MIRROR)/gmp
 GMP_SOURCE = gmp-$(GMP_VERSION).tar.xz
 GMP_INSTALL_STAGING = YES
-GMP_LICENSE = LGPLv3+
-GMP_LICENSE_FILES = COPYING.LESSERv3
+GMP_LICENSE = LGPLv3+ or GPLv2+
+GMP_LICENSE_FILES = COPYING.LESSERv3 COPYINGv2
 GMP_DEPENDENCIES = host-m4
 HOST_GMP_DEPENDENCIES = host-m4