shells/fish: fix build on ARM / POWER

error[E0308]: mismatched types
   --> src/path.rs:749:13
    |
748 |         let remoteness = remoteness_via_statfs(
    |                          --------------------- arguments to this function are incorrect
749 |             libc::statfs,
    |             ^^^^^^^^^^^^ expected fn pointer, found fn item
    |
    = note: expected fn pointer `unsafe extern "C" fn(*const i8, _) -> _`
                  found fn item `unsafe extern "C" fn(*const u8, _) -> _ {libc::statfs}`
note: function defined here
   --> src/path.rs:712:12
    |
712 |         fn remoteness_via_statfs<StatFS, Flags>(
    |            ^^^^^^^^^^^^^^^^^^^^^
713 |             statfn: unsafe extern "C" fn(*const i8, *mut StatFS) -> libc::c_int,
    |             -------------------------------------------------------------------

error[E0308]: mismatched types
   --> src/path.rs:725:34
    |
725 |             if unsafe { (statfn)(path.as_ptr(), buf.as_mut_ptr()) } < 0 {
    |                         -------- ^^^^^^^^^^^^^ expected `*const i8`, found `*const u8`
    |                         |
    |                         arguments to this function are incorrect
    |
    = note: expected raw pointer `*const i8`
               found raw pointer `*const u8`