Allow module-specific protoc flags.

You can add local protoc flags via LOCAL_PROTOC_FLAGS.

Change-Id: If246a4edafe1edcec6a7f193bde8ce3c83de745b
diff --git a/core/definitions.mk b/core/definitions.mk
index c236c0e..c1a627b 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -829,6 +829,7 @@
 $(hide) $(PROTOC) \
 	$(addprefix --proto_path=, $(PRIVATE_PROTO_INCLUDES)) \
 	$(PRIVATE_PROTO_JAVA_OUTPUT_OPTION)=$(PRIVATE_PROTO_JAVA_OUTPUT_DIR) \
+	$(PRIVATE_PROTOC_FLAGS) \
 	$(PRIVATE_PROTO_SRC_FILES)
 $(hide) touch $@
 endef
@@ -841,6 +842,7 @@
 @echo "Protoc: $@ <= $<"
 $(hide) $(PROTOC) \
 	$(addprefix --proto_path=, $(PRIVATE_PROTO_INCLUDES)) \
+	$(PRIVATE_PROTOC_FLAGS) \
 	--cpp_out=$(PRIVATE_PROTO_CC_OUTPUT_DIR) $<
 endef