Add missing require-effective-target lto

This prevents failure of an lto test in configurations
missing LTO support, such as VxWorks for kernel mode.

2020-11-02  Olivier Hainque  <hainque@adacore.com>

gcc/testsuite/
	* gcc.dg/tree-ssa/pr71077.c: Add
	dg-require-effective-target lto.

gcc/testsuite/gcc.dg/tree-ssa/pr71077.c

@@ -1,5 +1,6 @@
 /* PR c++/71077  */
 /* { dg-do link { target { i?86-*-* x86_64-*-* } } }  */
+/* { dg-require-effective-target lto } */
 /* { dg-options "-O3 -flto -march=core-avx2" }  */
 
 int *a;