bsd-user/main.c: Fix unused variable warning

On OpenBSD the compiler warns:
bsd-user/main.c:622:21: warning: variable 'sig' set but not used [-Wunused-but-set-variable]

This is because a lot of the signal delivery code is #if-0'd
out as unused. Reshuffle #ifdefs a bit to silence the warning.

(We make the minimum change here rather than removing all the
bsd-user patchset which should make this all work correctly and
there's no point giving them an awkward rebase task.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1500395194-21455-5-git-send-email-peter.maydell@linaro.org