p11-kit: Add nativesdk variant

p11-kit: Add nativesdk variant

The nativesdk variant is needed by the buildtools-tarball, when
p11-kit feature is enabled for gnutls. The error message is:

Missing or unbuildable dependency chain was: ['buildtools-tarball', 'nativesdk-wget', 'nativesdk-gnutls', 'nativesdk-p11-kit']

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Cc: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>