kmod-zfs-devel rpm should provide kmod-spl-devel

When configure is run with --with-spec=redhat, and rpms are built, the
kmod-zfs-devel package is missing

Provides: kmod-spl-devel = %{version}

which is required by software such as Lustre which builds against zfs
kmods. Adding it makes it easier for such software to build against
both zfs-0.7 (where SPL is separate and may be missing) and zfs-0.8.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #8930