Allow module-specific protoc flags.

You can add local protoc flags via LOCAL_PROTOC_FLAGS.

Change-Id: If246a4edafe1edcec6a7f193bde8ce3c83de745b
diff --git a/core/base_rules.mk b/core/base_rules.mk
index 132632b..7782c33 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -290,6 +290,7 @@
 else
 $(proto_java_sources_file_stamp): PRIVATE_PROTO_JAVA_OUTPUT_OPTION := --java_out
 endif
+$(proto_java_sources_file_stamp): PRIVATE_PROTOC_FLAGS := $(LOCAL_PROTOC_FLAGS)
 $(proto_java_sources_file_stamp) : $(proto_sources_fullpath) $(PROTOC)
 	$(call transform-proto-to-java)