pgsql: Avoid compiler warning in non-assert builds

Avoid compiler warning in non-assert builds

After 01575ad788e3, expand_single_inheritance_child()'s parentOID
variable is read only in an Assert, provoking a compiler warning in
non-assert builds.  Fix that by marking the variable with
PG_USED_FOR_ASSERTS_ONLY.

Per report and suggestion from David Rowley

Discussion: https://postgr.es/m/CAApHDvpjA_8Wxu4DCTRVAvPxC9atwMe6N%2ByvrcGsgb7mrfdpJA%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1f06b7fc6e2bc505bea046ca54a4555ca835d7c9

Modified Files
--------------
src/backend/optimizer/util/inherit.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)