package/volk: fix licensing information

Since 2.5.1, volk has been undergoing a relicensing from GPL-3.0+ to the
LGPL-3.0+, which is still not completely done 5by some large margin), so
it is still covered by the GPL-3.0+, but has parts already covered by
the LGPL-3.0+.

We so far only listed GPL-3.0+, but we also had the license file for the
LGPL-3.0+.

Add LGPL-3.0+ to the list of licenses; the COPYING file is the text of
the GPL-3.0+, so we already had it listed (note: the package has a
COPYING-GPL, but i's a symlink to COPYING).

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[yann.morin.1998@free.fr:
  - split out to its own patch
  - explain why we don't add COPYING-GPL
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>