Fix build with new googletest

The new googletest requires prefix to be non-empty.

Change-Id: I3ebd13861c4803b53dbeb4ac5f247984814bef45
diff --git a/health/2.1/vts/functional/VtsHalHealthV2_1TargetTest.cpp b/health/2.1/vts/functional/VtsHalHealthV2_1TargetTest.cpp
index 7df4926..da9f5bb 100644
--- a/health/2.1/vts/functional/VtsHalHealthV2_1TargetTest.cpp
+++ b/health/2.1/vts/functional/VtsHalHealthV2_1TargetTest.cpp
@@ -252,7 +252,7 @@
 }
 
 INSTANTIATE_TEST_SUITE_P(
-        , HealthHidlTest,
+        PerInstance, HealthHidlTest,
         testing::ValuesIn(android::hardware::getAllHalInstanceNames(IHealth::descriptor)),
         android::hardware::PrintInstanceNameToString);