fix(timerfd): actually compile the sys_timerfd module (export timerfd_* symbols)

The sys_timerfd module (timerfd_create/settime/gettime) existed on disk but was
never declared in header/mod.rs, so its code was NOT compiled and the symbols
were absent from libc — timerfd was entirely non-functional. The generated
header still declared the functions, so C programs compiled but failed to LINK
with 'undefined reference to timerfd_create/timerfd_settime' (e.g. libwayland's
event-loop.c, blocking the whole Wayland stack). Declare the module so the
implementation is built and the symbols exported.

src/header/mod.rs

@@ -174,6 +174,7 @@ pub mod sys_syscall;
 pub mod sys_time;
 #[deprecated]
 pub mod sys_timeb;
+pub mod sys_timerfd;
 //pub mod sys_times;
 pub mod arch_aarch64_user;
 pub mod arch_riscv64_user;