Pull in tableoid for inheiritance with rowMarks

As noted by Etsuro Fujita [1] and Dean Rasheed[2],
cb1ca4d8 changed ExecBuildAuxRowMark()
to always look for the tableoid in the target list, but didn't also
change preprocess_targetlist() to always include the tableoid.  This
resulted in errors with soon-to-be-added RLS with inheritance tests,
and errors when using inheritance with foreign tables.

Authors: Etsuro Fujita and Dean Rasheed (independently)

Minor word-smithing on the comments by me.

[1] 552CF0B6.8010006@lab.ntt.co.jp
[2] CAEZATCVmFUfUOwwhnBTcgi6AquyjQ0-1fyKd0T3xBWJvn+xsFA@mail.gmail.com