licensing: fix license info for Crypto/{bn,ec}

This code was part of Dolphin's relicensing from v2 to v2+ a while back,
we just never updated these copyright headers. I double-checked that
segher gave us permission to relicense this code to v2+ on 2015-05-16.

Source/Core/Common/Crypto/bn.cpp

@@ -1,6 +1,5 @@
 // Copyright 2007,2008  Segher Boessenkool  <segher@kernel.crashing.org>
-// Licensed under the terms of the GNU GPL, version 2
-// http://www.gnu.org/licenses/old-licenses/gpl-2.0.txt
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 #include <cstdio>
 #include <cstring>