hidl-gen: virtual inheritance from super interface

Bug: 32291558
Test: compiles/hidl_test
Change-Id: I56e3a65d784c4fdbca0d95e8ba30626e6aaa0f5c
diff --git a/generateCpp.cpp b/generateCpp.cpp
index fdca7fc..2c06990 100644
--- a/generateCpp.cpp
+++ b/generateCpp.cpp
@@ -171,7 +171,7 @@
         if (superType == NULL) {
             out << " : virtual public RefBase";
         } else {
-            out << " : public "
+            out << " : virtual public "
                 << superType->fullName();
         }