Function gio_sys::g_file_load_bytes

pub unsafe extern "C" fn g_file_load_bytes(
    file: *mut GFile, 
    cancellable: *mut GCancellable, 
    etag_out: *mut *mut c_char, 
    error: *mut *mut GError
) -> *mut GBytes

This is supported on crate feature v2_56 only.