unused-but-set-variable warning on REL_13_STABLE

My compiler emits the following warning on REL_13_STABLE when assertions
are disabled:

	reorderbuffer.c:2471:8: warning: variable 'spilled' set but not used [-Wunused-but-set-variable]
	 2471 |         Size            spilled = 0;
	      |                         ^

Adding PG_USED_FOR_ASSERTS_ONLY (as in the attached patch) seems sufficient
to suppress this warning.  On newer versions, this variable is used for
more than assertions, so the patch only needs to be applied to v13.  Since
this is trivial, I plan to commit it shortly.

-- 
nathan