Fix the build. Oops.

(We remove the declaration for on_exit() from the C library headers,
so do the same for the C++ <cstdlib>)

Change-Id: I31a21d5643068cd160724908dd8156863a44d95a
diff --git a/libstdc++/include/cstdlib b/libstdc++/include/cstdlib
index e520543..bb6f5a5 100644
--- a/libstdc++/include/cstdlib
+++ b/libstdc++/include/cstdlib
@@ -42,7 +42,9 @@
 using ::exit;
 using ::abort;
 using ::atexit;
+#if 0 /* MISSING FROM BIONIC */
 using ::on_exit;
+#endif
 
 using ::getenv;
 using ::putenv;