relibc: add syncfs() syscall binding

Added syncfs(fd) which calls fsync(fd) — equivalent on Red Bear
since all filesystem I/O is synchronous. Cross-referenced with
Linux 7.1 fs/sync.c.

src/platform/redox/mod.rs

@@ -1426,6 +1426,11 @@ impl Pal for Sys {
         Ok(())
     }
 
+    fn syncfs(fd: c_int) -> Result<()> {
+        // Red Bear filesystem I/O is synchronous — fsync(fd) is sufficient.
+        Self::fsync(fd)
+    }
+
     fn timer_create(clock_id: clockid_t, evp: &sigevent, mut timerid: Out<timer_t>) -> Result<()> {
         if evp.sigev_notify == SIGEV_THREAD {
             if evp.sigev_notify_function.is_none() {