libstdc++: Compile PR93978 testcase with -Wall

Now that the frontend issue PR c++/94038 is thoroughly fixed, the
testcase for PR93978 no longer fails to compile with -O -Wall, so add
-Wall to the testcase's compile flags to help ensure we don't regress
here.

libstdc++-v3/ChangeLog:

	PR libstdc++/93978
	* testsuite/std/ranges/adaptors/93978.cc: Add -Wall to
	dg-additional-options.  Avoid unused-but-set-variable warning.