unconvert warns about cgo helper file

[mdempsky]

Run unconvert on a file like

package p

// unsigned int myfunc() {}
import "C"

func f() interface{} {
	return C.myfunc
}

and you get a warning like

/home/mdempsky/.cache/go-build/d8/d81f94c4432b3a1654386c81604bbadaa821007a82fab40c6528acc37bbb19e6-d:38:56: unnecessary conversion

This is because cgo generates a line like:

var _Cfpvar_fp_myfunc unsafe.Pointer = (unsafe.Pointer)(unsafe.Pointer(&__cgo_myfunc))

I should either not warn about the file (it does have a "// Code generated by cmd/cgo; DO NOT EDIT." comment; see also #17), or maybe fix cmd/cgo to not emit the unnecessary conversion.