ART: Forward-declare AllocatorType (iwyu)

Test: mmma art
Change-Id: I064a1664727dfbed6ae91bde1658c7ba021bcc94
diff --git a/runtime/mirror/string-alloc-inl.h b/runtime/mirror/string-alloc-inl.h
index 7215c39..32b6bb4 100644
--- a/runtime/mirror/string-alloc-inl.h
+++ b/runtime/mirror/string-alloc-inl.h
@@ -24,6 +24,7 @@
 #include "base/bit_utils.h"
 #include "class.h"
 #include "class_root.h"
+#include "gc/allocator_type.h"
 #include "gc/heap-inl.h"
 #include "obj_ptr.h"
 #include "runtime.h"