Either use my compiled version in the repository I linked to or cross-compile them yourself from the source at https://github.com/OnionIoT/i2c-exp-driver. I found that you can't link against the binary file provided by the package for the onion (e.g. installable by opkg update && opkg install libonioni2c), due to some weird errors / symbol stripping whatever. So I compiled my own repo of header and library files at https://github.com/gamer-cndg/omega2-libs. Note that for your cross-compiled program to be able to run on an Omega, it will have to have all dependencies installed, such as libonioni2c.