security/mbedtls: fix LICENSE (dual with GPLv2+)

"Unless specifically indicated otherwise in a file, Mbed TLS files are provided
under the Apache License 2.0, or the GNU General Public License v2.0 or later
(SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later)."

Approved by: portmgr@ (blanket metadata change approval)