Add filter_init to prerequisites

Bug: 27954979
Change-Id: I03172a26906d0a1380546974df71a00719dd619e
diff --git a/extensions/libext.mk b/extensions/libext.mk
index 7aba36a..060ecf0 100644
--- a/extensions/libext.mk
+++ b/extensions/libext.mk
@@ -52,7 +52,7 @@
 ${MY_gen_lib_sources}: PRIVATE_CUSTOM_TOOL = $(PRIVATE_PATH)/filter_init $(PRIVATE_PATH)/$(notdir $@) > $@
 ${MY_gen_lib_sources}: PRIVATE_MODULE := $(LOCAL_MODULE)
 ${MY_gen_lib_sources}: PRIVATE_C_INCLUDES := $(LOCAL_C_INCLUDES)
-${MY_gen_lib_sources}: ${MY_gen}/% : $(LOCAL_PATH)/%
+${MY_gen_lib_sources}: ${MY_gen}/% : $(LOCAL_PATH)/% $(LOCAL_PATH)/filter_init
 	$(transform-generated-source)
 
 $(MY_GEN_INITEXT): $(MY_gen_lib_sources)