Fixup struct callback spec to work on 1.8, 1.9
Fixup struct callback spec to work on 1.8, 1.9

yghannam/ffi,ffi/ffi,sparkchaser/ffi,tduehr/ffi,ferventcoder/ffi,MikaelSmith/ffi,mvz/ffi,ferventcoder/ffi,yghannam/ffi,MikaelSmith/ffi,yghannam/ffi,tduehr/ffi,MikaelSmith/ffi,yghannam/ffi,sparkchaser/ffi,ffi/ffi,MikaelSmith/ffi,sparkchaser/ffi,sparkchaser/ffi,majioa/ffi,mvz/ffi,majioa/ffi,yghannam/ffi,ffi/ffi,mvz/ffi,ferventcoder/ffi,ferventcoder/ffi,tduehr/ffi,mvz/ffi,majioa/ffi,majioa/ffi,tduehr/ffi