Include third party licenses in dist tarballs

Since the merge of the Linux Solaris Porting Layer source tree into
the ZFS codebase, ZFS is now a double-licensed codebase, with the
former SPL codebase retaining its license (GPLv2+) within the ZFS
source tree.

However, the license files for SPL were not being included in the
tarballs generated by autotools. This change corrects that.

In addition, all the other third party licenses in the codebase are
now properly declared to be included in the dist tarballs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Neal Gompa <ngompa@datto.com>
Closes #8242