[tablet_copy_client-test] fix compiler warning

Fixed compilation warning in TabletCopyClientTest:

  src/kudu/tserver/tablet_copy_client-test.cc:149:16: \
      warning: 'GenerateTestData' overrides a member function but \
      is not marked 'override' [-Winconsistent-missing-override]
    virtual void GenerateTestData() {
                 ^
  src/kudu/tserver/tablet_copy-test-base.h:113:16: \
      note: overridden virtual function is here
    virtual void GenerateTestData() {
                 ^
  1 warning generated.

This patch does not contain any functional modifications.

Change-Id: Ib0194589f7ff38a66dc2ae85ff02f8f9346b1db6
Reviewed-on: http://gerrit.cloudera.org:8080/13674
Tested-by: Alexey Serbin <[email protected]>
Reviewed-by: Andrew Wong <[email protected]>

src/kudu/tserver/tablet_copy_client-test.cc

@@ -146,7 +146,7 @@ class TabletCopyClientTest : public TabletCopyTest {
   Status CompareFileContents(const string& path1, const string& path2);
 
   // Injection of 'supports_live_row_count' modifiers through polymorphic characteristic.
-  virtual void GenerateTestData() {
+  void GenerateTestData() override {
     Random rand(SeedRandom());
     NO_FATALS(tablet_replica_->tablet_metadata()->
         set_supports_live_row_count_for_tests(rand.Next() % 2));