Fix compilation failure when v8 compiled with GCC 5.x due to `-Werror=unused-variable`

test/unittests/compiler/linkage-tail-call-unittest.cc

Index: test/unittests/compiler/linkage-tail-call-unittest.cc
diff --git a/test/unittests/compiler/linkage-tail-call-unittest.cc b/test/unittests/compiler/linkage-tail-call-unittest.cc
index fc5930af80144c987494a0f16847d390a92c04a8..741021a4462b9dfc1494b6f3226e274876163d92 100644
--- a/test/unittests/compiler/linkage-tail-call-unittest.cc
+++ b/test/unittests/compiler/linkage-tail-call-unittest.cc
@@ -26,6 +26,7 @@ class LinkageTailCall : public TestWithZone {
   CallDescriptor* NewStandardCallDescriptor(LocationSignature* locations) {
     DCHECK(arraysize(kMachineTypes) >=
            locations->return_count() + locations->parameter_count());
+    USE(kMachineTypes);
     return new (zone()) CallDescriptor(
         CallDescriptor::kCallCodeObject, MachineType::AnyTagged(),
         LinkageLocation::ForAnyRegister(MachineType::Pointer()),