checkpatch: allow SPDX-License-Identifier

According to: https://spdx.org/ids-how

, let's still allow QEMU to use
the SPDX license identifier:

// SPDX-License-Identifier: ***

CC: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20190426062705.4651-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>