Fix bitness issues

[panekj]

Replaces i64 with isize for ffi functions that operate on time_t which is c_long that is either i32 or i64 based on the platform, this way it can cleanly cast between types

Example signature (copied from generated bindings):

pub type __time_t = ::std::os::raw::c_long;
pub type __suseconds_t = ::std::os::raw::c_long;
pub type __socklen_t = ::std::os::raw::c_uint;
pub type time_t = __time_t;
[...]
pub printf_datetime_tags: ::std::option::Option<
        unsafe extern "C" fn(
            buffer: *mut t_gui_buffer,
            date: time_t,
            date_usec: ::std::os::raw::c_int,
            tags: *const ::std::os::raw::c_char,
            message: *const ::std::os::raw::c_char,
            ...
        ),
    >,

Fixes poljar/weechat-matrix-rs#84
Fixes #34

[panekj]

Updated header comes from weechat/weechat@80def4a

[poljar]

Looks sensible.