qemu.inc: Add seccomp PACKAGECONFIG option

Add the seccomp PACKAGECONFIG option to allow building seccomp features
in QEMU. The libseccomp library is available in additional layers (e.g.
meta-security).

Additionally this serves as a way to disable seccomp by default to avoid
the configure of QEMU automatically finding it (via pkg-config) on the
build host when building qemu-system-native and auto enabling the
feature.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Ruslan Babayev <fib@cisco.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>