local.conf.sample: Add sdl to nativesdk qemu PACKAGECONFIG

Enable SDL by default for nativesdk as we do with natve, this will
allow qemu with ADT to use graphics no vnc by default.

[YOCTO #6446]

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>