Fix compilation on aarch64 Linux

[ankane]

Changes i8 to c_char to match the bindings (as c_char is u8 on aarch64 Linux).

Otherwise, compilation fails with many instances of:

error[E0308]: mismatched types
   --> src/file.rs:193:17
    |
191 |             ffi::oneWriteLine(
    |             ----------------- arguments to this function are incorrect
192 |                 self.ptr,
193 |                 line_type as i8,
    |                 ^^^^^^^^^^^^^^^ expected `u8`, found `i8`
    |

This currently prevents building impg on aarch64 Linux.