libstdc++: Fix invalid order in PSTL inplace_merge test [PR90276]

This looks like a typo in the upstream test that causes a failure in
debug mode. It has been reported upstream.

libstdc++-v3/ChangeLog:

	PR libstdc++/90276
	* testsuite/25_algorithms/pstl/alg_merge/inplace_merge.cc: Fix
	comparison function to use less-than instead of equality.