Add getsid stub

src/platform/linux/mod.rs

@@ -267,6 +267,10 @@ impl Pal for Sys {
         e(syscall!(GETRLIMIT, resource, rlim)) as c_int
     }
 
+    fn getsid(pid: pid_t) -> pid_t {
+        e(unsafe { syscall!(GETSID, pid) }) as pid_t
+    }
+
     fn gettid() -> pid_t {
         e(unsafe { syscall!(GETTID) }) as pid_t
     }