include/exec/poison: Mark CONFIG_SOFTMMU as poisoned

CONFIG_SOFTMMU should never be used in common code, so mark
it as poisoned, too.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1498454578-18709-6-git-send-email-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
