ART: Forward-declare AllocatorType (iwyu)

Test: mmma art
Change-Id: I064a1664727dfbed6ae91bde1658c7ba021bcc94
diff --git a/runtime/mirror/class.h b/runtime/mirror/class.h
index d09b484..f2ba7d0 100644
--- a/runtime/mirror/class.h
+++ b/runtime/mirror/class.h
@@ -26,7 +26,6 @@
 #include "dex/dex_file_types.h"
 #include "dex/modifiers.h"
 #include "dex/primitive.h"
-#include "gc/allocator_type.h"
 #include "object.h"
 #include "object_array.h"
 #include "read_barrier_option.h"
@@ -38,6 +37,10 @@
 class TypeList;
 }  // namespace dex
 
+namespace gc {
+enum AllocatorType : char;
+}  // namespace gc
+
 namespace hiddenapi {
 class AccessContext;
 }  // namespace hiddenapi