c2rust-refactor: Structurally match types more conservatively by comparing visibility

[ahomescu]

This fixes issues where reorganize_definitions would match internal structure definitions like statvfs against the types in libc which keep some fields as private:

    pub struct statvfs {
        pub f_bsize: c_ulong,
        pub f_frsize: c_ulong,
        pub f_blocks: crate::fsblkcnt_t,
        pub f_bfree: crate::fsblkcnt_t,
        pub f_bavail: crate::fsblkcnt_t,
        pub f_files: crate::fsfilcnt_t,
        pub f_ffree: crate::fsfilcnt_t,
        pub f_favail: crate::fsfilcnt_t,
        pub f_fsid: c_ulong,
        pub f_flag: c_ulong,
        pub f_namemax: c_ulong,
        __f_spare: [c_int; 6],
    }

[fw-immunant]

I was initially wondering how we translate definitions from libc into types that have some private fields when afaik c2rust always emits all fields as public. But I realize you mean that c2rust-refactor was comparing transpiled structs with definitions from the actual libc crate, which is not transpiled.

LGTM.