Fix build with new googletest

The new googletest requires prefix to be non-empty.

Change-Id: I3ebd13861c4803b53dbeb4ac5f247984814bef45
diff --git a/vibrator/aidl/vts/VtsHalVibratorTargetTest.cpp b/vibrator/aidl/vts/VtsHalVibratorTargetTest.cpp
index b0e6c72..d2977c6 100644
--- a/vibrator/aidl/vts/VtsHalVibratorTargetTest.cpp
+++ b/vibrator/aidl/vts/VtsHalVibratorTargetTest.cpp
@@ -228,7 +228,7 @@
     }
 }
 
-INSTANTIATE_TEST_SUITE_P(, VibratorAidl,
+INSTANTIATE_TEST_SUITE_P(Vibrator, VibratorAidl,
                          testing::ValuesIn(android::getAidlHalInstanceNames(IVibrator::descriptor)),
                          android::PrintInstanceNameToString);