libstdc++: testsuite: adjust name_fortify test for pre-defined _FORTIFY_SOURCE

Otherwise we get failures with toolchains that have _FORTIFY_SOURCE
defined already to a different value like 3.

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names_fortify.cc: Undefine _FORTIFY_SOURCE.

libstdc++-v3/testsuite/17_intro/names_fortify.cc

 // { dg-do compile { target *-*-linux* } }
 // { dg-add-options no_pch }
 
+#undef _FORTIFY_SOURCE
 #define _FORTIFY_SOURCE 2
 // Now we can define the macros to poison uses of non-reserved names:
 #include "names.cc"