tests: require working signbit

tests: require working signbit

* modules/ceilf-tests (Depends-on): Add signbit.
* modules/ceill-tests (Depends-on): Likewise.
* modules/floorf-tests (Depends-on): Likewise.
* modules/floorl-tests (Depends-on): Likewise.
* modules/round-tests (Depends-on): Likewise.
* modules/roundf-tests (Depends-on): Likewise.
* modules/roundl-tests (Depends-on): Likewise.
* modules/trunc-tests (Depends-on): Likewise.
* modules/truncf-tests (Depends-on): Likewise.
* modules/truncl-tests (Depends-on): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>