README : Using jextract

[lauric.desauw at tutanota.com]

Hello, 

I'm planning on using jextract for a project and I was following the "Using jextract" example from your README as a starting point. 

I did not succeed to make it works. (attached is the code I used).  
In RuntimeHelper.java:63, in `downcallHandle(String name, FunctionDescriptor fdesc)`
The `SYMBOL_LOOKUP` never find the symbol of the linked function (distance in the example). 

I checked in debug, the `SYMBOL_LOOKUP` does contain the classes linked. 

Maybe I'm doing something wrong. If you have any insight I would be happy to take them. 
I precised the detail of the configuration and the commands used in the README attached. 
I stay available if you have any question. 

Best regards--
Lauric Desauw