Allow to also use generate mojo with existing native src files

This allows to use the maven plugin also with existing JNI code while not using hawtjni to generate it at all.

We would love to make use of this in the netty project to ship our native code ASAP. So it would be awesome if you would consider to include this patch and do a release :)

Thanks for your hard work on this project. It really rocks!

Here you can see the change in action:
https://github.com/netty/netty/blob/native-hawtjni/transport-native-epoll/pom.xml