Add libaio-devel BuildRequires

The zfs-test package needs a build requirement on the libaio-devel
package. Without it ./configure will correctly determine that
mmap_libaio cannot be built and it will be skipped.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7821
Closes #7824