Commit c7f419f5 moved softmmu-only files out of the root
directory, but forgot to move the trace events, which should
no longer be generated to "trace-root.h". Fix that by adding
softmmu/trace-events.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Claudio Fontana <cfontana@suse.de>
Reviewed-by: Claudio Fontana <cfontana@suse.de>
Message-id: 20200805130221.24487-1-philmd@redhat.com

[Rebased onto meson.
--Stefan]

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>