libstdc++: Require c++98_only effective target for a test

This test checks a conversion which only exists in C++98 and won't
compile since C++11. It uses { dg-options "-std=gnu++98" } so that it is
explicitly run in C++98 mode. This change also adds a target selector so
that the test will be skipped if the dg-options directive is filtered
out or overridden.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_ios/conv/voidptr.cc: Add c++98_only
	target selector.

libstdc++-v3/testsuite/27_io/basic_ios/conv/voidptr.cc

@@ -16,6 +16,7 @@
 // <http://www.gnu.org/licenses/>.
 
 // { dg-options "-std=gnu++98" }
+// { dg-do run { target c++98_only } }
 
 #include <ios>
 #include <testsuite_hooks.h>