Add include guard for Gtest.h

Only allow one include per translation unit, else there are duplicate
symbol definitions.

Test: It compiles
Change-Id: I5e88e7a21d2a630f9d674c552afc9e1165092a41
diff --git a/libaidlvintf_test_helper/include/aidl/Gtest.h b/libaidlvintf_test_helper/include/aidl/Gtest.h
index d88877e..bde89a3 100644
--- a/libaidlvintf_test_helper/include/aidl/Gtest.h
+++ b/libaidlvintf_test_helper/include/aidl/Gtest.h
@@ -13,6 +13,8 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+#pragma once
+
 #include <gtest/gtest.h>
 
 namespace android {