Fix libaio-devel requirement for Debian-based distributions

BuildRequires tags for "-devel" packages in the RPM spec file do not
work when building on Debian-based distributions.

Fix this issue by making this requirement conditional to RPM-based
distributions.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #7829
Closes #7831