systemd: add group sgx to udev package

>From NEWS for v250:
* Device nodes for the Software Guard eXtension enclaves (sgx_vepc) are
  now also owned by the system group "sgx".

>From NEWS for v248:
* Intel SGX enclave device nodes (which expose a security feature of
  newer Intel CPUs) will now be owned by a new system group "sgx".

Fixes following journal error entry during startup:
  /lib/udev/rules.d/50-udev-default.rules:43 Unknown group 'sgx', ignoring
This is seen already on kirkstone.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bab455cd)
Signed-off-by: Steve Sakoman <steve@sakoman.com>