Require storage type on enum.

Test: hidl_test
Bug: 33197891
Change-Id: I055fa058f4a9fc0daa0a87af4ae8b0b229665a61
diff --git a/EnumType.h b/EnumType.h
index 36ac968..ccdf384 100644
--- a/EnumType.h
+++ b/EnumType.h
@@ -30,7 +30,7 @@
 struct EnumType : public Scope {
     EnumType(const char *localName,
              const Location &location,
-             Type *storageType = NULL);
+             Type *storageType);
 
     const Type *storageType() const;
     const std::vector<EnumValue *> &values() const;