BUG/MINOR: threads: fix soft-stop without multithreading support

BUG/MINOR: threads: fix soft-stop without multithreading support

When thread support is disabled ("USE_THREAD=" or "USE_THREAD=0" when
building), soft-stop doesn't work as haproxy never ends after stopping
the proxies.

This used to work fine in the past but suddenly stopped working with
ef422ced91 ("MEDIUM: thread: make stopping_threads per-group and add
stopping_tgroups") because the "break;" instruction under the stopping
condition is never executed when support for multithreading is disabled.

To fix the issue, let's add an "else" block to run the "break;"
instruction when USE_THREAD is not defined.

It should be backported up to 2.8