idmap2: use 'override' where possible

Sprinkle idmap2 with the override specifier to help the compiler catch
future errors.

Test: builds
Change-Id: I6932c5a80ac607f310e256620194e04acfd19ffd
diff --git a/cmds/idmap2/tests/TestHelpers.h b/cmds/idmap2/tests/TestHelpers.h
index adea329..a7c2f28 100644
--- a/cmds/idmap2/tests/TestHelpers.h
+++ b/cmds/idmap2/tests/TestHelpers.h
@@ -123,7 +123,7 @@
 
 class Idmap2Tests : public testing::Test {
  protected:
-  virtual void SetUp() {
+  void SetUp() override {
 #ifdef __ANDROID__
     tmp_dir_path_ = "/data/local/tmp/idmap2-tests-XXXXXX";
 #else
@@ -136,7 +136,7 @@
     idmap_path_ = tmp_dir_path_ + "/a.idmap";
   }
 
-  virtual void TearDown() {
+  void TearDown() override {
     EXPECT_EQ(rmdir(tmp_dir_path_.c_str()), 0)
         << "Failed to remove temporary directory " << tmp_dir_path_ << ": " << strerror(errno);
   }