Add install-includes field to cabal file

[vmchale]

This adds an install-include field to the library, so that

cabal new-sdist

now works correctly.

[vincenthz]

Thanks, but those headers are private and should remain private, install-include is for exposing C headers

[vmchale]

Ah okay, my apologies. I have added them to the extra-source-files instead.

[vincenthz]

Thanks !