[libc++][modules] Fix disabling Unicode (#81294)

-DLIBCXX_ENABLE_UNICODE=OFF or -D_LIBCPP_HAS_NO_UNICODE doesn't build
without this change.
NOKEYCHECK=True
GitOrigin-RevId: 30cd1838dc334775f7a29f57b581f2bdda3f0ea1

Author: poyaoc97

modules/std/ostream.inc

@@ -33,8 +33,10 @@ export namespace std {
   using std::println;
 
   using std::vprint_nonunicode;
+#    ifndef _LIBCPP_HAS_NO_UNICODE
   using std::vprint_unicode;
-#  endif // _LIBCPP_STD_VER >= 23
+#    endif // _LIBCPP_HAS_NO_UNICODE
+#  endif   // _LIBCPP_STD_VER >= 23
 
 #endif // _LIBCPP_HAS_NO_LOCALIZATION
 } // namespace std