rb: add cbindgen config for sys_timerfd

sys_timerfd was fully implemented in Rust (timerfd_create,
timerfd_settime, timerfd_gettime, itimerspec support) but lacked
the cbindgen.toml needed to generate the <sys/timerfd.h> C header.
Qt6's QSocketNotifier and many other C programs depend on this header.
Without it, cbindgen skips the directory entirely.

This adds the cbindgen config + trailer with all function prototypes,
matching the pattern used by sys_signalfd.

src/header/sys_timerfd/cbindgen.toml

@@ -0,0 +1,38 @@
+# Linux kernel timerfd_create/timerfd_settime/timerfd_gettime
+# See <https://man7.org/linux/man-pages/man2/timerfd_create.2.html>.
+#
+# This header is non-POSIX (a Linux/glibc extension) but fully implemented
+# in relibc. It is required by Qt6 for QSocketNotifier and many other
+# C programs that use timer file descriptors.
+#
+# Includes the itimerspec from <time.h>.
+sys_includes = ["signal.h", "stdint.h", "stddef.h"]
+include_guard = "_SYS_TIMERFD_H"
+trailer = """
+#ifndef TFD_CLOEXEC
+#define TFD_CLOEXEC 0x80000
+#endif
+
+#ifndef TFD_NONBLOCK
+#define TFD_NONBLOCK 0x800
+#endif
+
+#ifndef TFD_TIMER_ABSTIME
+#define TFD_TIMER_ABSTIME 0x1
+#endif
+
+#ifndef TFD_TIMER_CANCEL_ON_SET
+#define TFD_TIMER_CANCEL_ON_SET 0x2
+#endif
+
+int timerfd_create(int clockid, int flags);
+int timerfd_settime(int fd, int flags, const struct itimerspec *new_value, struct itimerspec *old_value);
+int timerfd_gettime(int fd, struct itimerspec *curr_value);
+"""
+language = "C"
+style = "Tag"
+no_includes = true
+cpp_compat = true
+
+[enum]
+prefix_with_name = true