configure: make fortify_source=yes by default

Commit c87ea116 "configure: add --without-default-feature"
changed fortify_source from "yes" to "$default_feature". But there's
no option to turn it on, we only turn it off for debug build.  I think
this should always be initialized to "yes" in the first place.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Message-Id: <20220422100825.3692002-1-mjt@msgid.tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>