remove backtrace() prototype that don't exists under

- openbsd
- bitrig
- netbsd

src/unix/bsd/openbsdlike/mod.rs

@@ -361,7 +361,6 @@ extern {
     #[cfg_attr(target_os = "netbsd", link_name = "__clock_gettime50")]
     pub fn clock_gettime(clk_id: ::c_int, tp: *mut ::timespec) -> ::c_int;
     pub fn __errno() -> *mut ::c_int;
-    pub fn backtrace(buf: *mut *mut ::c_void, sz: ::size_t) -> ::size_t;
     pub fn shm_open(name: *const ::c_char, oflag: ::c_int, mode: ::mode_t)
                     -> ::c_int;
     pub fn pthread_main_np() -> ::c_int;