libstdc++: Add missing target selector to std::expected test

This test should use a target selector of c++23 so that the explicit
-std=gnu++23 option can be removed, to allow testing with later
standards too.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/expected/bad.cc: Add missing target
	selector.

libstdc++-v3/testsuite/20_util/expected/bad.cc

@@ -1,5 +1,5 @@
 // { dg-options "-std=gnu++23" }
-// { dg-do compile }
+// { dg-do compile { target c++23 } }
 
 #include <expected>