[foreign-jextract] No code generated for structs inside union

[Filip Krakowski]

Hi,

it seems that jextract does not generate code for structs declared 
within unions. Tested with "ibv_send_wr" shown on the following man page.

https://linux.die.net/man/3/ibv_post_send

Although it is possible to get a MemorySegment for "wr", it's rather 
difficult to work with it without having its corresponding MemoryLayout. 
In this case it is relatively important to access the fields of the 
structs in the union, as these describe how an operation is to be executed.

Best regards,
Filip