test: fix cctest -Wunused-variable warning

Backport-PR-URL: #19176
PR-URL: #18530
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

Author: bnoordhuis

test/cctest/test_node_postmortem_metadata.cc

@@ -130,7 +130,7 @@ TEST_F(DebugSymbolsTest, ReqWrapList) {
   // NOTE (mmarchini): Workaround to fix failing tests on ARM64 machines with
   // older GCC. Should be removed once we upgrade the GCC version used on our
   // ARM64 CI machinies.
-  for (auto it : *(*env)->req_wrap_queue()) {}
+  for (auto it : *(*env)->req_wrap_queue()) (void) ⁢
 
   auto queue = reinterpret_cast<uintptr_t>((*env)->req_wrap_queue());
   auto head = queue +