Fix build with new googletest

The new googletest requires prefix to be non-empty.

Change-Id: I3ebd13861c4803b53dbeb4ac5f247984814bef45
diff --git a/boot/1.1/vts/functional/VtsHalBootV1_1TargetTest.cpp b/boot/1.1/vts/functional/VtsHalBootV1_1TargetTest.cpp
index fba9a5e..7c58ef3 100644
--- a/boot/1.1/vts/functional/VtsHalBootV1_1TargetTest.cpp
+++ b/boot/1.1/vts/functional/VtsHalBootV1_1TargetTest.cpp
@@ -81,6 +81,6 @@
 }
 
 INSTANTIATE_TEST_SUITE_P(
-        , BootHidlTest,
+        PerInstance, BootHidlTest,
         testing::ValuesIn(android::hardware::getAllHalInstanceNames(IBootControl::descriptor)),
         android::hardware::PrintInstanceNameToString);