Add mising AAPT2 dependency

Bug: 33381544
Test: m clean; mma -j with a prebuilt aar using aapt2
Change-Id: Ie41def84aa35f3774abd6d713fb4a75d63a88876
diff --git a/core/aapt2.mk b/core/aapt2.mk
index ccc4535..e34b09b 100644
--- a/core/aapt2.mk
+++ b/core/aapt2.mk
@@ -33,7 +33,7 @@
 ifneq ($(my_generated_res_dirs),)
 my_generated_resources_flata := $(my_compiled_res_base_dir)/gen_res.flata
 $(my_generated_resources_flata): PRIVATE_SOURCE_RES_DIRS := $(my_generated_res_dirs)
-$(my_generated_resources_flata) : $(my_generated_res_dirs_deps)
+$(my_generated_resources_flata) : $(my_generated_res_dirs_deps) $(AAPT2)
 	@echo "AAPT2 compile $@ <- $(PRIVATE_SOURCE_RES_DIRS)"
 	$(call aapt2-compile-resource-dirs)