NthItemTest: Add definition for virtual destructor

This fixes a compiler warning from clang:

unittest/nthitem_test.cc:22:7: warning:
 'NthItemTest' has no out-of-line virtual method definitions;
 its vtable will be emitted in every translation unit [-Wweak-vtables]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

unittest/nthitem_test.cc

@@ -21,6 +21,7 @@ int test_data[] = { 8, 1, 2, -4, 7, 9, 65536, 4, 9, 0, -32767, 6, 7};
 // The fixture for testing GenericHeap and DoublePtr.
 class NthItemTest : public testing::Test {
  public:
+  virtual ~NthItemTest();
   // Pushes the test data onto the KDVector.
   void PushTestData(KDVector* v) {
     for (int i = 0; i < ARRAYSIZE(test_data); ++i) {
@@ -30,6 +31,11 @@ class NthItemTest : public testing::Test {
   }
 };
 
+// Destructor.
+// It is defined here, so the compiler can create a single vtable
+// instead of a weak vtable (fixes compiler warning).
+NthItemTest::~NthItemTest() = default;
+
 // Tests basic results.
 TEST_F(NthItemTest, GeneralTest) {
   KDVector v;