deadcode should not consider functions with valid //export comments to be unused

[BenLubar]

Example: https://gist.github.com/BenLubar/851d3de63699f50e76ef0f866a0049a3

Since deadcode probably isn't going to be able to parse cgo very easily, it's best to just assume that every function with //export exactFunctionName on the line before it is used if any Go file in the package imports the C package.