configure: enable debug-mutex if debug enabled

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180425025459.5258-5-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>