Make verify_compact_attribute available in non-assert builds

Make verify_compact_attribute available in non-assert builds

6f3820f37 adjusted the assert-enabled validation of the CompactAttribute
to call a new external function to perform the validation.  That commit
made it so the function was only available when building with
USE_ASSERT_CHECKING, and because TupleDescCompactAttr() is a static
inline function, the call to verify_compact_attribute() was compiled
into any extension which uses TupleDescCompactAttr().  This caused issues
for such extensions when loading the assert-enabled extension into
PostgreSQL versions without asserts enabled due to that function being
unavailable in core.

To fix this, make verify_compact_attribute() available unconditionally,
but make it do nothing unless building with USE_ASSERT_CHECKING.

Author: Andrew Kane <andrew@ankane.org>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAOdR5yHfMEMW00XGo=v1zCVUS6Huq2UehXdvKnwtXPTcZwXhmg@mail.gmail.com