Extended messages cannot have required fields (must be optional).

There is an error when I use the proto file *db_query_record_msg.proto*:

_db_query_record_msg.proto:36:11: Message extensions cannot have required fields.
/home/quix0r/git/seeks/src/plugins/query_capture/Makefile:850: recipe for target 'db_query_record_msg.pb.cc' failed
make: *** [db_query_record_msg.pb.cc] Error 1_

This small change fixes it.