x11/rlaunch: fix build with unsigned char

error[E0308]: mismatched types
   --> src/x11.rs:310:54
    |
310 |             (self.xlib.XLookupString)(&mut keyevent, &mut c_char, 1, null_mut(), null_mut());
    |             -------------------------                ^^^^^^^^^^^ expected `u8`, found `i8`
    |             |
    |             arguments to this function are incorrect
    |
    = note:    expected raw pointer `*mut u8`
            found mutable reference `&mut i8`