test: prevent guard-breaker optimization with LTO

In case of very aggressive optimizations the compiler can
optimize guard-breaker function away and the `unit/guard`
test would fail.

test/unit/guard.cc

@@ -13,7 +13,11 @@ static int __attribute__((noinline))
 stack_break_f(char *ptr)
 {
 	char block[2048];
-	char sum = 0;
+	/*
+	 * Make sum volatile to prevent a compiler from
+	 * optimizing away call to this function.
+	 */
+	volatile char sum = 0;
 	memset(block, 0xff, 2048);
 	sum += block[block[4]];
 	ptrdiff_t stack_diff = ptr > block ? ptr - block : block - ptr;